Allow user to install different versions of monit

This cookbook will be a lot more flexible if users can choose which version of monit they want to install. My work thus far while includes the ability to install monit from src while maintaining backwards compatibility (i.e. installs via "package" by default). These changes work well for Debian >= 6.0.

**Note:** I think it's also useful for a user to be able to target a specific package version. This makes it easy to install a newer version of monit (e.g. assuming they have APT source entries for backports, testing, or experimental repos) without having to add a pinning preference.

Remaining tasks:
* round out support for other OSs (centos, scientific, etc...)
* allow user to specify what version of the package they want to install

Thoughts?