Improve save/load, also add save screenshot

Save/load now gets its filename from the current map, also added a button that lets you save a complete screenshot of everything on the three canvases (note that this cannot be loaded). 

![screen shot 2013-12-23 at 18 06 38](https://f.cloud.github.com/assets/3536982/1801989/d6c72768-6bf4-11e3-934b-a192f0e7fd19.png)