Check that regex actually matched

This was cluttering up my javascript console with:

    Uncaught TypeError: Cannot read property '1' of null

on every page I viewed which didn't have feedburner junk at the end of
its URL.
Bump manifest_version to 2.

There are no changes which affect this extension, but this is needed for
modern Chromes to load the extension in developer mode. The changes in
version 2 are documented at
http://developer.chrome.com/extensions/manifestVersion.html#manifest-v1-changes