Add a test framework to HTTPS Everywhere

Zack (redwire)'s goog. summer of code project involves adding a lot of important update handling code that needs to be tested. Now is a good time to start adding tests for the main extension code.

This pull requests adds a basic script to run tests (`./test.sh`) and a few unit tests. It uses the testing framework from the add-on SDK, which can access XPCOM. So we can just run `cfx test` using a Firefox profile that has HTTPS Everywhere installed and import the HTTPS Everywhere component.