Make 'disabled' MUST_USE_ATTRIBUTE for compatibility with CSS [disabled] selectors

[lrowe]

When a ReactDOMComponent is created with the property disabled: true subsequently setting the property to disabled: false the HTML attribute disabled="true" was being left in the DOM.

Example: http://jsfiddle.net/kb3gN/409/

[zpao]

Nice! I did a minimal test to make sure it still works with <input>s (where the attribute actually has meaning) - http://jsfiddle.net/zpao/HF4mV/. But we should be making sure we still work with CSS selectors like this (which actually seems like a great way to test this whole module...)