Feature/gbr 35

This is a pull request  for https://github.com/caitp/grunt-bower-release/issues/35

We only delete tags and suffix them if ```options.overwriteVersion === true```.

Also, this pull request features unit tests for the git endpoint which are using the Sinon.JS mocking framework. I remember I've seen a request to add tests for endpoints in https://github.com/caitp/grunt-bower-release/issues/4. It doesn't cover it entirely, but covers the new functionality.

There is also a need for proper end-to-end tests and I'll submit a separate feature request for it.

In addition, I've added the ```async``` library as ```grunt.util.async``` is now considered to be deprecated (see http://gruntjs.com/api/grunt.util#grunt.util.async).