Add XML style JSX namespacing support using @jsxns docblock comment.

Adds support for using XML namespaces in JSX to refer to custom defined identifiers in a `@jsxns` docblock comment. Can make a namespace refer to any JS identifier.

```javascript
/**
 * @jsx React.DOM
 * @jsxns {"UI": "MyApp.UI", "External": "External"}
 */
```

Should address most of the concerns and patterns mentioned in #221.

Included some basic tests for validating namespace definition and usage.