Fix Review Bot when SITE_ROOT is used.

When the extension would pass the Review Board instance url to the
workers it was not including SITE_ROOT. This would cause Review Bot to
fail when Review Board was not installed at the root of a domain.

The SITE_ROOT is now included in the generated urls.

Closes #6

Ran a Review Board development server with both SITE_ROOT='/', and SITE_ROOT='/rb/'. Review Bot functioned properly in both instances.