Add burndown training graph as a widget to the second dashboard

**Please do not merge yet.**

- Split this new graph into another dashboard.
- Retrieve data from a Google Spreadsheet for the burndown statistics.
- Make a graph out of this data.
- Style the graph widget.

I still need to work out how to make the key for the lines display, and possibly make the lines slightly thicker again so they're more readable. There's also a slight CSS problem where the August month on the x axis creeps over the edge of the dashboard widget - can I request that @fofr possibly take a look? This is just about visible in this screenshot:
![screen shot 2014-05-13 at 15 13 15](https://cloud.githubusercontent.com/assets/355033/2958350/c59921bc-daa8-11e3-8ce5-fdf8760f26db.png)

Are the colours of the lines and the background OK? I just used the defaults.