Write unit tests

[nemtsov]

To make editing and refactoring safer:

• Write unit tests to cover the different use-cases
• Ensure tests run when npm test is called
• Ensure a failure of a single test will return npm test with a non-zero error code

[nemtsov]

@qm3ster are you, by any chance, interested in taking this one?

[qm3ster]

I almost did this by myself, but then decided not to.
Shouldn't they be integration tests though?
(run real express, do the example, check return)

[nemtsov]

Shouldn't they be integration tests though?

That sounds even better.