Feature : map-type directive support

This PR adds support for a new directive `map-type`, to use Google API [Map Types](https://developers.google.com/maps/documentation/javascript/maptypes) objects.

This enable the directive to overlay images or HTML elements on specific areas, using a tiles server or any source of data. Alternatively, this is possible to use map types as selectable "base" type in the control menu (aka single layer).

The PR includes few jasmine tests based on the proposed current architecture, and some documentation/examples.
You can [check the documentation online](http://rawgit.com/Antwan86/angular-google-maps/feature/maptype-support/example/overlay-map-type.html).

Feel free to discuss/improve. Let me know things are required prior to merging this PR.