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
"is_logstash_plugin" sounds like a question, maybe use ruby's predicate-style naming? ".logstash_plugin?(...)"

This method also seems to do two things:
- reads a gem file
- returns nil if it's not a valid logstash plugin

Can we split these two actions?