Plugin manager

Plugin manager for the new plugin structure

 - Install
  - From Rubygems
    - Latest
     - Specific version
  - From gem file

 - List installed plugins
   - All
   - Specific name
   - Specific group

 - Uninstall a plugin

 - Update
   - Single plugin to latest version
   - Single plugin to specific version
   - All to latest version
The group part is only optional.
So its either the plugin name, or group.
If the group option is given, the plugin name is automatically set to nil to ensure all the plugins are considered when the group check is done.