Add capybara-webkit for evaluation
Run only tagged specs with selenium

Capybara-webkit is now the default javascript driver.
You can still use selenium for you spec by tagging it with selenium or
by setting the environment variable SELENIUM to something truthy
A couple more Selenium-only specs
These pass locally but fail in CI
tag failing spec as selenium's
Tag addtional spec with selenium
Merge pull request #548 from Tahi-project/selenium_to_capybara