Module for dom property config

I broke out the `DefaultDOMPropertyConfig.js` file into a module called `dom-property-config`

 - Added a thorough README for `dom-property-config`
 - Added package.json boilerplate
 - Added a grunt task as a generic function for building a module
 - Updated the Gruntfile.js

The biggest value add in this change is

 - A really useful small part of React is available as a standalone module
 - Purely build based, no changes to development approach for React authors
 - A hidden internal now has a high quality piece of documentation.

This may be a good approach for other React internals. The biggest value of this approach is the ability to write a high quality targeted README.md for an internal piece of the React engine.

cc @petehunt @zpao