add Magit hook to git-gutter:update-hooks

If the user has Magit, the buffer will update when a change is made.

If the user doesn't have Magit, then nothing changes.  Emacs will create
a hook magit-revert-buffer-hook that is never run.
Merge pull request #68 from jschaf/magit-hook

add Magit hook to git-gutter:update-hooks
Bump up version and update Changes
Fix indentation
Re-implement staging hunk command by reference to git-gutter+
Refactoring
Fix bug staging hunk bug

Staging hunk command does not work files which are in not top directory of
repository. And add test of hunk command utility function.
Merge pull request #69 from syohex/improve-staging-hunk

Improve staging hunk
Bump up version and update Changes