Add code coverage reporting

Using [node-coveralls](https://github.com/cainus/node-coveralls).

We're currently at [![Coverage Status](https://coveralls.io/repos/wikimedia/restbase/badge.png?branch=coverage)](https://coveralls.io/r/wikimedia/restbase?branch=coverage) -- not a bad start!
Don't dig into internal `node_modules` directories. Packages register their binaries in `package.json`, which are aliased from `node_modules/.bin`. E.g. `node_module/.bin/_mocha`.

And there's no need to access that either since that's present in the PATH for package `scripts` by default. Plain `_mocha` and `coveralls` will work here.