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.
Make sure computeProperties() runs when expected, fixes #12.
put same version number in package.json and bower.json
Observable doc.
fix Evented API doc, fixes #18
Observable doc update.
Add documentation index, fixes #16.
remove empty keys.js file checked in by mistake, fixes #20
original Stateful doc
changes from watch() to observe() + general updates
add YAML header
Stateful documentation
spelling fixes
document Destroyable
document Evented
document sniff