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
Use just 'proxy' not '@proxy'.

Also, if you're doing conditional `if proxy` then you don't need to set `:default => false` because the default is nil, and nil is also falsey, so your code still works.