expose google map type to python

can now set `plot.map_options.map_type` to

* `satellite`
* `terrain`
* `roadmap`
* `hybrid`

![screen shot 2014-09-11 at 10 28 29 pm](https://cloud.githubusercontent.com/assets/1078448/4245058/a0cf02ae-3a2d-11e4-95f5-c98840bf1d44.png)
`