Fix #54: Adding support for commit --amend with empty messages

This fixes #54 by building the commit command without the message
if the intent is to amend the current commit. The API would be
as follows:

      .pipe(git.commit("", {args: "--amend"} ))

*	modified:   lib/commit.js
- First verifying the combination of arguments and seeing if the
  --amend was passed before failing.

- Building the commit message with the options that amends the
 commit without calling the editor as described on #54.