Use the WebRequest API to redirect requests to the extension.

With the WebNavigation API, users could get stranded on an issue after
refreshing, since the extension redirect would leave an entry behind in
the history. WebRequest prevents this since it cancels the original
request from completing when it redirects.

Fixes #33
Merge pull request #39 from zetafunction/master

Use the WebRequest API to redirect requests to the extension.