[vector-api] Google Maps support (prototype)

This PR is opened for discussion. The attached code implements Google Maps support in ol3. It's not ready to be merged yet, but might be useful as a starting point for a better implementation.

Notes:

* The Google map is drawn by the Google Maps API into a `DIV`, so AIUI it is compliant with the Google Maps TOS (but IANAL).
* All navigation is handled by ol3.
* It only works (at the moment) with the DOM renderer.
* This DIV is inserted directly into the stack of per-layer DIVs managed by the DOM renderer.
* `pointer-events: none` is used to stop the Google Map from receiving, er, pointer events, I don't think this is yet supported in IE.
* It is not possible to disable animated zooming in Google Maps: Google have marked the request [WONTFIX](https://code.google.com/p/gmaps-api-issues/issues/detail?id=3033).
* In theory this could be back-ported to the `master` branch.

![ol3-google-maps](https://f.cloud.github.com/assets/6942/1785297/0bba7f4e-68e2-11e3-99c3-44e19ab7a95b.png)