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.
Sorry I don't get it. This is listing code, just for display purpose and this is not executed. The JS code is already inside a separate JS file. Should I find a way to include it anyway ? This is not convenient as I only highlight specifics parts of the code, and displaying the code actually executed won't be as clear.

Let me know your thoughts on this.