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
document Destroyable, Evented, Stateful, and sniff
Documentation updates.
Link updates, etc.
Stateful: standardize which property changes are reported.

Report changes to underscore props, but not to instance only props,
regardless of whether or not Object.observe() is natively supported.

Leverages the _getProps() method to filter what PropertyListObserver reports.
Also changes _getProps() to return a hash instead of an array.

Fixes #17, #22.
prepare for 0.2.0-beta tag
decor should require requirejs-dplugins with version 0.2.x, fixes #23
prepare for 0.2.0-dev tag
Fix broken link (replace old article by new one)
Invalidating.md: remove obsolete paragraph (fix broken link)
Gruntfile: added config for jsdoc's path format