Convert all unit tests to mocha syntax

This is a direct translation of the tests in tests/unit from nodeunit
format to mocha format.

The are all passing when NODE_ENV=testing
Switch make test-unit to run mocha
Remove old nodeunit package and tests

These have been superseded by the mocha tests in ./test
Remove reference to nodeunit in docs
Update npm-shrinkwrap.json

This removes nodeunit from the shrinkwrap. Mocha and supertest haven't
been added because they are 'devDependencies' and therefore don't need
to be installed in the production environment.
Run jshint against mocha's test directory