Changed zen project URL

This is the new URL: https://github.com/zen-kernel/zen-kernel, github also redirects to that
sys-kernel/geek-sources: added zen USE for geek-sources-3.17.2.ebuild
eclass: exFAT zen-kernel URL updated
Merge pull request #109 from loxdegio/patch-1

Updated zen-kernel project URL + readded zen USE to geek-sources-3.17.2