Fixes #880 - Adds support for sending tags in gelf output

This adds tag support to the gelf output.

It adds a new option called ship_tags, set to true by default, when
sends all tags as a new field called _tags to GELF. Tags are converted
from an array to a comma separated list.
Updated CHANGELOG
Merge pull request #333 from jamtur01/gelftags

Fixes #880 - Adds support for sending tags in gelf output