Added plainmap function to just draw a map.

Can now use gmaps.plainmap() to draw a Google map with no overlay.
The rationale is that it will be easier to develop the documentation
pipeline using this as a test model, rather than the more complex
heatmap.

All the views are in a single js file at the moment. This makes sense
for now, but it might be worth thinking about refactoring this in the
future.

The Python models will also need refactoring.
Added docstring to plainmap function.
Merge pull request #42 from pbugnion/simple_map

Add plain map widget.