readme: add flat badges and url variables

If you're running a next-gen web framework, you might as well stay ahead of the pack... with your badges.

__screenshot__:
![screen shot 2014-07-06 at 03 36 22](https://cloud.githubusercontent.com/assets/2467194/3487870/f9ec4f1e-04ad-11e4-9d3e-1f5ce167f56d.png)