Capistrano V3 Support

I've added support for capistrano version 3 and updated the README.  I've tested it a little bit and everything seems to work (deploying and rolling back).  I did my best to model it after the other V3 modules that the capistrano team is putting out.

One change that's worth mentioning is that the "on_rollback" functionality is gone in V3.  I'm not sure how it's being handled now (I'm assuming they handle it by calling the rollback rake tasks) but that changes some of the code you had.

Nothing for V2 has been touched.  Only new files so this should have zero impact on existing installations.