Support for the other styles from Google Maps

This adds support for other styles from Google Maps.

```python
gg_tiles = GoogleTiles(style="terrain")
```

Supported are `street` (default), `terrain`, `satellite`, and `only_streets` (tranparent streets layer).

I also added a short test and a small change to the .gitignore file.


![figure_1](https://f.cloud.github.com/assets/800487/2285154/9b813b42-9fd1-11e3-93ad-962f29740bb9.png)