Add Derelict::Plugin class
Add Derelict::Parser::PluginList
Add Derelict::Plugin::Manager
Add PluginManager#list
Add PluginManager#installed?
Add remaining plugin manipulation commands

Fixes #4.
Add external logging to plugin commands
Attempt to fix Coveralls on Ruby 1.8
Merge pull request #10 from bradfeehan/plugins

Add plugin manipulation commands