Consolidate JSON processing by using Jackson

The processing of JSON data (parsing and generating) in graylog2-server is currently done by two libraries, [JSON.simple](https://code.google.com/p/json-simple/) and [Gson](https://code.google.com/p/google-gson/).

In order to consolidate this part (and to get [some](https://github.com/FasterXML/jackson-module-afterburner) [nice](https://github.com/FasterXML/jackson-datatype-joda) [features](http://wiki.fasterxml.com/JacksonModuleGuava) like [integration with JAX-RS](https://github.com/FasterXML/jackson-jaxrs-json-provider) or a [streaming parser](http://wiki.fasterxml.com/JacksonInFiveMinutes#Streaming_API_Example) as well as a [possible speedup](http://blog.novoj.net/2012/02/05/json-java-parsers-generators-microbenchmark/)) this pull request replaces the two libraries with the well-known [Jackson](http://wiki.fasterxml.com/JacksonHome)) library.