enable specify git binary location along with other options in the constructor

Resolving this issue https://github.com/sentientwaffle/gift/issues/32

In the constructor I am passing in a git_options directory, which can specify the git binary location. Please let me know if you have any questions. Please see below for a sample use.

    git  = require './src/'
    repo = git '/Users/edaniya/git/gift', null, {bin: "/usr/bin/tmp/git"}
    repo.commits "master", 1, (err, commits) ->
      console.log commits