Cleaned up readme slightly and added info on using blanket.js coverage tool

I've cleaned up readme a bit and added a brief (maybe a bit too much so) description on how to use blanket.js coverage tool with node-coveralls. I'm using it myself successfully with [passport-reddit](https://github.com/Slotos/passport-reddit/blob/master/Makefile#L27), so it comes from fresh experience.

- Dima