[#6] Change testing framework to mocha 1.8.x.

Install mocha 1.8.x as a dev dependency, and replace all vows tests
with mocha. To run all tests, copy .test.env.example to .test.env,
edit as required, and run:

    foreman run -e .test.env mocha

or for a document format:

    foreman run -e .test.env mocha -R spec