Add CORS headers for the v2 API.

Create a decorator for adding Cross-Domain headers, then use it
to allow CORS requests from any domain in the `api2` app.
Add test coverage for @crossdomain decorator
Add coverage for api2 'week' parameter format.
Merge pull request #161 from mreid-moz/allow_cors_for_api2

Add CORS headers for the v2 API.