Support for the other styles from Google Maps

This adds support for other styles from Google Maps.


Supported are `street` (default), `terrain`, `satellite`, and `only_streets` (transparent streets layer).

I also added a short test and a small change to the .gitignore file.


**Short complete example:**
```python
import cartopy
from cartopy.io.img_tiles import GoogleTiles
import matplotlib.pylab as plt

plt.figure(figsize=(10, 10))
ax = plt.subplot(111, projection=cartopy.crs.PlateCarree())
ax.set_extent([12.0, 13.0, 47.0, 48.0])

gg_tiles = GoogleTiles(style="terrain")
ax.add_image(gg_tiles, 11)

plt.show()
```

![figure_1](https://f.cloud.github.com/assets/800487/2291944/3f9ce206-a04c-11e3-8e26-a78d9582e38a.png)