#93 Add a "Category" attribute to commands.

[Age Mooy]

The number of commands in Karma is growing and now that
we have command plugins that number is only going to
grow faster. It would be handy if it was possible to
group commands into "categories" or "groups" or whatever.

This would make the current help output much more
readible and future IDE implementations could also use
it to order commands into menus or a two-level tree-view.

So let's add a "category" attribute to the command
descriptor and add some util methods to the relevant
classes so you can get a list of commands by category.

[Arjen Smedes]

Logged In: YES
user_id=825315

Yep, like maven.