feature(graylog2 thrift): add GZIP support

This commit adds GZIP support to the scribe interface.

A new thrift endpoint LogCompressed taking compressed binary
messages has been added. The binary message is passed to
SimpleGELFClientHandler which takes care of the decompression
before parsing the JSON payload.

The original Log endpoint (taking an uncompressed JSON string
as input) is still present for backwards compatibility.