Added goal to the display name of extensions.
Added information to extensions
Information is shown in the body of each extension.
Showing them as helper (? icon) was an option, but it messed up the layout...
Moved files to subpackages
Files which are specific for Play 2.X (extensions for commands) were moved to subfolder.
The reason is to keep the code organized when new extensions are added for Play 1.X or Play 2.3, which will invoke sbt directly.
Recovered the 'Identifier' field in test-only command.