Add Integration Tests

[smcgregor]

This is intended to be an introductory issue, but we can always use more test cases :)

You can find information about "integration tests" on the priv.ly development guide. A good test to add is one that opens the URL of a public website that contains a privly link and checks that the extension injects it properly. Examples can be found here.

This issue is for helping people setup Selenium WebDriver. Comment if the documentation for Selenium WebDriver is not sufficient and you need help. We will then translate any issues or misconceptions into changes in the documentation.