Fixes #222 - add test for readable error condition.

- Cannot reproduce bug, added a test case for the specific situation though.
- The bug lines in the existing.py script don't line up, believe bug is
against earlier code.
- Garden some code and get it up to lint standards while in there.

lint

Add test for readable lib
Fixes #221 - Do not allow editing a url

- Makes the url field read-only when editing a bookmark
- Updates the css to make that look less sucky.
- In the end, most urls are copied, done from an extension, or from a
bookmarklet. Editing them doesn't make much sense. The pattern we suggest is
to remove/re-link so that we know it's new and can fetch content for it and
such.