Adding cookbook support for other init styles (just Upstart for now).

This adds an additional attribute that allows users to choose an init style. Runit remains the default, so backwards compatibility is preserved. I've added support for Upstart, but others could be easily implemented, as well.

Part of the motivation for this stems from also experiencing the intermittent crashing @joelmoss described in https://github.com/berkshelf/berkshelf-api/issues/121. 

Tested on Ubuntu 12.04 and 14.04.