exec-path-from-shell resets exec-path

This ensures that kibit-mode-path is added back to exec-path once
exec-path-from-shell-initialize is run. I have this in my init.d config
and it could be useful for others too.

If exec-path-from-shell isn't installed, this should do nothing.
Merge pull request #11 from wjlroe/fix-clash-with-shell-path

exec-path-from-shell resets exec-path