Define jenkins::plugin type for managing plugins

Manage Jenkins plugins with Boxen.

## Install a version of a plugin

    jenkins::plugin { 'notification':
       version => '1.4'
    }

## Remove a plugin

    jenkins::plugin { 'notification':
       ensure => absent
    }

## TODO

- [x] create jenkins::plugin type
- [ ] add specs for jenkins::plugin
- [ ] update README