Add a plugin resource/provider

This adds a `jenkins_plugin` resource/provider.  This allows adding and removing plugins with:

```ruby
    # Install the latest version of greenballs
    jenkins_plugin 'greenballs'

    # Install version 1.2 of ant
    jenkins_plugin 'ant' do
      action :install
      version '1.2'
    end

    # Uninstall the envinject plugin
    jenkins_plugin 'envinject' do
      action :remove
    end
```

If you specify an explicit version, the plugin will be updated (thus making this a possible replacement for https://github.com/opscode-cookbooks/jenkins/pull/73).  It also pins plugins which allows updating system plugins.

NOTE:  If you set version to `latest` (default), the latest version will be installed, but the plugin will never be updated.  For now, you should use explicit versions to update plugins.

This is backwards compatible with the `node['jenkins']['server']['plugins']` attribute.

Fixes [COOK-3623](https://tickets.opscode.com/browse/COOK-3623)