Update Injector and create injectable object factory

- Using e.g. Factory<Converter> instead of the injector directly makes its purpose more specific and disallows any future abuse of the injector's functions

src/main/java/fr/xephi/authme/initialization/factory/Factory.java

@@ -0,0 +1,19 @@
+package fr.xephi.authme.initialization.factory;
+
+/**
+ * Injectable factory that creates new instances of a certain type.
+ *
+ * @param <P> the parent type to which the factory is limited to
+ */
+public interface Factory<P> {
+
+    /**
+     * Creates an instance of the given class.
+     *
+     * @param clazz the class to instantiate
+     * @param <C> the class type
+     * @return new instance of the class
+     */
+    <C extends P> C newInstance(Class<C> clazz);
+
+}

src/main/java/fr/xephi/authme/initialization/factory/FactoryDependencyHandler.java

@@ -0,0 +1,46 @@
+package fr.xephi.authme.initialization.factory;
+
+import ch.jalu.injector.Injector;
+import ch.jalu.injector.context.ResolvedInstantiationContext;
+import ch.jalu.injector.handlers.dependency.DependencyHandler;
+import ch.jalu.injector.handlers.instantiation.DependencyDescription;
+import ch.jalu.injector.utils.ReflectionUtils;
+
+/**
+ * Dependency handler that builds {@link Factory} objects.
+ */
+public class FactoryDependencyHandler implements DependencyHandler {
+
+    @Override
+    public Object resolveValue(ResolvedInstantiationContext<?> context, DependencyDescription dependencyDescription) {
+        if (dependencyDescription.getType() == Factory.class) {
+            Class<?> genericType = ReflectionUtils.getGenericType(dependencyDescription.getGenericType());
+            if (genericType == null) {
+                throw new IllegalStateException("Factory fields must have concrete generic type. " +
+                    "Cannot get generic type for field in '" + context.getMappedClass() + "'");
+            }
+
+            return new FactoryImpl<>(genericType, context.getInjector());
+        }
+        return null;
+    }
+
+    private static final class FactoryImpl<P> implements Factory<P> {
+
+        private final Injector injector;
+        private final Class<P> parentClass;
+
+        FactoryImpl(Class<P> parentClass, Injector injector) {
+            this.parentClass = parentClass;
+            this.injector = injector;
+        }
+
+        @Override
+        public <C extends P> C newInstance(Class<C> clazz) {
+            if (parentClass.isAssignableFrom(clazz)) {
+                return injector.newInstance(clazz);
+            }
+            throw new IllegalArgumentException(clazz + " not child of " + parentClass);
+        }
+    }
+}