Make it possible to change the URL of the server.

Otherwise, webtest isn't able to change the URL of the server. With this
changes, it's possible to do, in the TestCase class, something like this:

    self.app.proxy.uri = 'http://new-server'

And the requests will be directed to the specified new server.
Add some documentation about server adress change.
Add a simpler API with "self.app.server_url"
Merge pull request #17 from mozilla-services/change-webtest-uri

Make it possible to change the URL of the server.