Service injector - implement stricter requirements for PostConstruct methods

- Implement similar restrictions as prescribed by the PostConstruct documentation:
   - Class may have at most one method annotated with PostConstruct
   - PostConstruct method must return void
- Javadoc: replace mentions of injector construction where any injection method was meant

src/test/java/fr/xephi/authme/initialization/AuthMeServiceInitializerTest.java

@@ -166,7 +166,7 @@ public class AuthMeServiceInitializerTest {
         PostConstructTestClass testClass = initializer.get(PostConstructTestClass.class);
 
         // then
-        assertThat(testClass.werePostConstructsCalled(), equalTo(true));
+        assertThat(testClass.wasPostConstructCalled(), equalTo(true));
         assertThat(testClass.getBetaManager(), not(nullValue()));
     }
 
@@ -188,6 +188,18 @@ public class AuthMeServiceInitializerTest {
         initializer.get(InvalidPostConstruct.ThrowsException.class);
     }
 
+    @Test(expected = RuntimeException.class)
+    public void shouldThrowForMultiplePostConstructMethods() {
+        // given / when / then
+        initializer.get(InvalidPostConstruct.MultiplePostConstructs.class);
+    }
+
+    @Test(expected = RuntimeException.class)
+    public void shouldThrowForPostConstructNotReturningVoid() {
+        // given / when / then
+        initializer.get(InvalidPostConstruct.NotVoidReturnType.class);
+    }
+
     @Test(expected = RuntimeException.class)
     public void shouldThrowForAbstractNonRegisteredDependency() {
         // given / when / then

src/test/java/fr/xephi/authme/initialization/samples/InvalidPostConstruct.java

@@ -9,10 +9,8 @@ import javax.inject.Inject;
 public abstract class InvalidPostConstruct {
 
     public static final class WithParams {
-        @SuppressWarnings("unused")
         @Inject
         private AlphaService alphaService;
-        @SuppressWarnings("unused")
         @Inject
         private ProvidedClass providedClass;
 
@@ -41,4 +39,28 @@ public abstract class InvalidPostConstruct {
             throw new IllegalStateException("Exception in post construct");
         }
     }
+
+    public static final class NotVoidReturnType {
+        @Inject
+        private ProvidedClass providedClass;
+
+        @PostConstruct
+        public int returnsInt() {
+            return 42;
+        }
+    }
+
+    public static final class MultiplePostConstructs {
+        @Inject
+        private ProvidedClass providedClass;
+
+        @PostConstruct
+        public void postConstruct1() {
+            // --
+        }
+        @PostConstruct
+        public void postConstruct2() {
+            // --
+        }
+    }
 }

src/test/java/fr/xephi/authme/initialization/samples/PostConstructTestClass.java

@@ -17,22 +17,15 @@ public class PostConstructTestClass implements SettingsDependent {
     @Inject
     private BetaManager betaManager;
     private boolean wasPostConstructCalled = false;
-    private boolean wasSecondPostConstructCalled = false;
     private boolean wasReloaded = false;
 
     @PostConstruct
-    protected void setFieldToTrue() {
+    public void postConstructMethod() {
         wasPostConstructCalled = true;
     }
 
-    @PostConstruct
-    public int otherPostConstructMethod() {
-        wasSecondPostConstructCalled = true;
-        return 42;
-    }
-
-    public boolean werePostConstructsCalled() {
-        return wasPostConstructCalled && wasSecondPostConstructCalled;
+    public boolean wasPostConstructCalled() {
+        return wasPostConstructCalled;
     }
 
     public BetaManager getBetaManager() {