add local hostname feature

Add ability to specify a local hostname instead of graypy just using
gethostname().

Use: accesslog2gelf.py --localname me

This feature currently requires a patched version of graypy
(https://github.com/iggy/graypy), but I have already sent upstream a pull
request. Hopefully the patch gets in a released version soon.

Sponsored by bubbleup.net

Signed-off-by: Brian Jackson <iggy@theiggy.com>