Merge pull request #3 from sabof/master

Remove the symbols when the minor mode is being disabled
Merge pull request #4 from sabof/master

stop processing, when buffer doesn't have a file name
Bump up version and update Changes
Bump up version and Update Changes(Thanks sabof)
return 128 if default-directory does not exist

Invoking call-process-shell-command in a non-existing default-directory
will throw error.
Merge pull request #5 from doitian/quiet-non-existing-default-directory-error

return 128 if default-directory does not exist
Bump up version and Update Changes(Thanks doitian)
fix changes
Fixed #6

Add '--no-ext-diff' for not using external diff
Bump up version and update Changes
Make git-gutter:lighter customization variable.
Use make-local-variable instead of make-variable-buffer-local
Add git-gutter to "change-major-mode-hook"
Add 'git-gutter:lighter' explanation.
Bump up version and update Changes
Don't change window-margin if file has no changes.
Add about update change information
Update version and update Changes
Add hook to window-configuration-change-hook.

Fixed problem that 'git diff' is not shown when switching buffer.
Bump up version and update Changes