Enhancement: Introduce Linting

We love clean code. To help us write the cleanest and finest
JavaScript, we've introduced Linting with the help of the JSHint
library.

See the [JSHint documentation](http://www.jshint.com/docs/) for
more information.

What we learned:
* Linting
* JSHint, how to configure it
Refactor: Fix code following JSHint's report

JSHint told us to fix a few bits in our code. Check.
Update README from commit history

Generated using the following command:

`git log --format="## %s%n%n%b%n[View this commit](https://github.com/mattwynne/js-testing-training/commit/%H)%n" --reverse > README.md`