Add the possiblity to add plugins the same way we add hooks.

This commit aims to allow a user to add plugins the same way one add
hooks.

Please see #7 as it depends on it
Merge pull request #9 from Spredzy/add_plugin_define

Add the possiblity to add plugins the same way we add hooks.
Add a folder define to add gerrit extra folders

At installation time, gerrit creates a set of few folders.
It can uses more than the one created (ie. hooks, plugins, etc...).
This commits allow one to specify those folders.