Make decor/Evented module to replace dojo/Evented.

There are a lot of ways to make this module.  I copied the technique from
dojo, but we could alternately:

1. emit events on the document itself, and then use
   addEventListener()/removeEventListener() rather than dcl.advise()
2. hand craft the code to keep track of who is listening for what event

Fixes #4.
Auto-chain Invalidating.computeProperties() and refreshRendering().
Fixes #11.
Move Invalidating.md to decor.