Add jscoverage support

To generate coverage html report:
`$ npm run coverage`
This will generate *coverage.html* in the project root. We can also reconfigure it to generate json coverage data and send it to something like [Coveralls](https://coveralls.io).

In the future, every *test.js* file must contain the following line which determines whether to run the instrumented or original code:
```js
var libDir = process.env.COVERAGE ? '../lib-cov/' : '../lib/';
```
... and everytime a project file is `require`d, the path must be relative to `libDir` (does not apply to dependencies installed via **NPM**):
```js
var googleapis = require(libDir + 'googleapis.js');
```

Closes #30.