make faces inherit from git-gutter

Rather than having identical definition in 2 places, just have
git-gutter-fr faces inherit from their git-gutter counterparts. This
will also allow theme designers to support both modes more easily
without breaking anyone's personal changes if they've defined the fringe
faces and not the git-gutter versions (or for some reason actually want
to have different values for the two different modes).
Merge pull request #13 from tanzoniteblack/inherit-faces

make faces inherit from git-gutter