Adds coveralls code badge

This adds the coveralls code coverage badge to the `README.md`. You can see what this looks like on my fork: https://github.com/chesleybrown/knex

It works by pushing the code coverage results to coveralls after the travis build is completed.

A contributor to this repo will have to add the project to https://coveralls.io for it to work though.

Minor Note: I updated the Travis build badge to be SVG as the SVG badge is much clearer. I'm also using the SVG for the coveralls code coverage badge.