pkg.installed support for USE flags, overlay and keyword accept

As in title I have extended the ebuild.py module to allow the user to specify wanted USE flags directly in pkg.installed state function.
It is possible to specify the preferred overlay. It is also possible to force accept_keyword an the give package.
I've implemented some functions in pkg_resources.py which wraps ebuild.py functions in a similar way to the one I've used for "ex_mod_init", so they could also be used for other OS/Distro (FreeBSD ?)
I've updated doc strings accordingly so it should be clear how to use the new functionalities.

Thanks to @terminalmage for pointing me in the right direction about pkg_resources.py