Fix the test pages for sign up and log in

The new sign up and log in pages are minimalist, not having the
features that BasePage provides. To remove errors and element
search waits, this adds a core page with globally necessary entities.
Also, Captcha and Confirm Password fields were removed, some elements
were renamed and a new error element introduced, so the tests
now recognise these.
Merge pull request #250 from zanata/test-minimalist-signup

Fix the test pages for sign up and log in