implemented plugin that runs RSpec tests

Hi,

I have implemented a new plugin that runs RSpec tests and raises individual warning events for each failed test (as well as a single critical event).

I took inspiration from the [ServerSpec check](https://github.com/sensu/sensu-community-plugins/blob/master/plugins/serverspec/check-serverspec.rb) but with some notable improvements:

- both the `Ruby` and `RSpec` binaries can be specified (particularly useful if you use [Omnibus](https://github.com/opscode/omnibus)).
- you can pass in environment variables easily

My company has been using this plugin across our AWS infrastructure for about a week now so any notable issues have been ironed out.