Add description to all trees by default.

[rwjblue]

We should add a default description to all of our generated trees to make the slow tree output more useful. Broccoli tries to grab the description based on the constructors name, but that is somewhat error prone and does not lead to really nice names. We can do better. 😃

Examples:

ef4/broccoli-sourcemap-concat#1
ember-cli/broccoli-es6modules#2
ember-cli/ember-cli#2857