add local hostname option

Added an option to GELFHandler/make_message_dict to pass a locally specified
hostname as the source of the message.

This will be used by a patched (that hopefully gets merged) version of
accesslog2gelf.py (https://github.com/knyar/apache2gelf - patched version:
https://github.com/iggy/apache2gelf).

Signed-off-by: Brian Jackson <iggy@theiggy.com>