Added some extra checks for document location. Fixes #6.

I couldn't reproduce the circumstances that caused this error, but it
is easy enough to fix and shouldn't have unwanted side effects (famous
last words).
User admin pages now have good links to profile pages. Fixes issue #5
Drop support for getpersonas.com, and some cleanup. Fixes issue #7

Now I only need to handle AMO and MXR, so I did some refactoring to
simplify the regular expressions.