Processor Cache Keys

Implementation for #671.

Processors (engines or transformers) MAY define a method called `cache_key` that returns a static cache key identifying the state of the function. (Method name maybe revised)

* [ ] Cleanup `uri => processor cache key` function
* [ ] Needs test cases

/cc @chancancode @elia 
So if you upgrade `coffee-script-source`, that should automatically bust any asset caches.