Use `tree.annotation` if present.

[rwjblue]

Newer versions of broccoli use annotation instead of description (see broccoli-plugin documentation).

Without including annotation in our getDescription code, we do not have the ability to see the proper descriptions for nodes based on broccoli-plugin that do not also add a .description (which some do manually).

[rwjblue]

I've prepared a before/after example which is somewhat annoying to review in full detail:

before

after

[rwjblue]

Updated to add some unit tests around getDescription.