Add plugin for configuring job with hipchat
Add base class for plugin settings which requires subclasses to define #configure
Add PluginCollection class
Initialize plugin collection when initializing a job
Add Job#add_plugin and #remove_plugin
Call configure on plugin collection when building freestyle config
Move InvalidType exception definition to avoid load order issues
Verify collection is only issue with PluginSettings::Base objects
Fix nesting of xml documents
Add missing tag
Merge branch 'feature/extract-plugins'