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
this in effect will test (if version == false !! version == nil) && version == nil 

since `version` was assigned from `@version` and that undefined instance variables value is `nil` I think this could be simply:

```ruby
version = Gem::Requirement.default if version.nil?
```

or better yet,

```ruby
version ||= Gem::Requirement.default
```