Move tests to mocha

Background

We want to get rid of nodeunit and it has been evaluated that mocha is an adequate choice.

Issue #14 (closed) already changed the assertion library.

What to change

• Add mocha as a dependency
• Change the core tests to BDD with mocha
• Rework the browser tests harness to work with mocha and change the test to BDD with mocha.

Hints for testers

• Running npm test for adblockpluscore should be successful.
• CI should be green.