Use git diff instead of diff.

As mentioned in the README, Git on Windows doesn't add diff to path.
But it's not even needed, because git diff can compare any 2 files.
As a bonus, it also allows to use --text-diff and --patience.
st3 version
st2 compat
Make grammar scope more specific
ugly hack for gutter icons in st3
Quick fix for issue #41
Merge branch 'master' of github.com:alisey/GitGutter into alisey
Merge branch 'master' of github.com:ccampbell/GitGutter into ccampbell
Merge branch 'master' of github.com:maelnor/GitGutter into maelnor
Merge remote-tracking branch 'refs/remotes/jisaacks/master'

Conflicts:
	git_gutter_handler.py
Catch LookupError on unknown encodings
Update the Windows section of the README
Merge pull request #43 from alisey/master

Update the Windows section of the README
Add alternatives section to README
Fix bug when deleted lines are close by ...

When 2 lines are removed with a 1 line gap between them (say 17 and 19) then the line between them (18) will try to be occupied by both the deleted_top and deleted_bottom icon. Only 1 icon is able to fill this position tho and looks ugly. So I added a new deleted_dual icon that is both deleted icons combined to use in this situation.
Fix some small parts I left out when added the dual icon
Fix bug - not running when initialized
PEP-8 formatting
allow git command to be set with 'git_gutter_git_command'
Use the encoding helper in diff

Uses the encoding helper for cleaning up encodings in the diff.