Add PostgreSQL support (#1620)

* Add PostgreSQL support

* Fix code issues and create integration tests

* Fix identation

* Test Postgres data source in postgres integration test

* Relocated the postgres driver

src/main/java/fr/xephi/authme/settings/properties/DatabaseSettings.java

@@ -10,7 +10,7 @@ import static ch.jalu.configme.properties.PropertyInitializer.newProperty;
 public final class DatabaseSettings implements SettingsHolder {
 
     @Comment({"What type of database do you want to use?",
-        "Valid values: SQLITE, MYSQL"})
+        "Valid values: SQLITE, MYSQL, POSTGRESQL"})
     public static final Property<DataSourceType> BACKEND =
         newProperty(DataSourceType.class, "DataSource.backend", DataSourceType.SQLITE);