Capistrano 3 support

While this does not replicate all of the current features of the Cap 2 version of this gem, it brings forward the most pertinent ones.

Features removed:

* `:jenkins_use_unstable`
* `:jenkins_scm_log_prefix`

Additionally, it makes use of JSON instead of XML, for reasons of ease of use & a decrease in data transfer needs (fewer bytes is better for API calls!)

It does lacks test entirely, which I will be adding in the near future, but if that is a showstopper, this can just remain open for now.