Redis bugfix and GelfSender flexibility

My changes/addons should be useful for your project:

- GelfRedisSender was not threadsafe. Now uses a JedisPool. The configuration of GelfRedisSender was also buggy

- For flexible plugin of other Sender-implementations I've refactored the GelfSenderFactory to be more flexible by the use of the Java ServiceLoader. Now you can plugin or replace new senders by an implementation of RedisGelfSenderProvider and configure them in any META-INF/services in the classpath
-> First effect of this refactoring: You don't need to explicit define the Test-Gelfsender in all your tests/TestLoggers. Instead of this, it's now configured under META-INF/services