Add selenium tests

Here's some Selenium test to partially address https://github.com/EFForg/privacybadgerchrome/issues/249.
To run the tests just: `cd tests && ./run_selenium_tests.sh`
Basically it packages the extension and launches a Selenium controlled browser with the freshly packed .crx.

The tests in selenium directory are mostly demonstrative. I'm planning to add more tests that assert conditions on HTTP requests/responses using ChromeDriver's [performance logging](https://sites.google.com/a/chromium.org/chromedriver/logging/performance-log) functionality.