Enabled programmatic attachment of custom GelfSenderProvider

GelfSenderFactory uses the Java ServiceProvider mechanism to plugin custom GelfSender implementations. In some situations this mechanism doesn't work in a satisfactory manner (e.g. it isn't possible to plugin in a provider in a JBoss7 Module because of the JBoss Modules custom classloader implementation).
For this case i extended the GelfSenderFactory to plugin custom GelfSenderProvider programmatically.