Add gemfury plugin for working with releases

Currently it's pretty cumbersome pushing new releases to gemfury.  Especially if we're releasing a beta version of a gem and are on a branch other than master.

The command looks something like:

`git push fury my-remote-branch-name-that-can-be-really-long:master`

There's no need to have to remember to do this.

Ideally the `grls` command would know if it was in a repo that released via Gemfury vs a repo that released via Rubygems.