Adds a "Report Problems" entry to the extension's context menu

Clicking on it takes the user to a form that's currently hosted at http://shumway-issues.tillschneidereit.net/input
Store all exceptions' messages, stacks and source (as in, rough part of the runtime) in the runtime
Send recorded exceptions as POST data when reporting errors
Prune duplicated exceptions before sending error reports
Make issue reporting facility more visible by adding it as a button that appears on hover over the Shumway overlay
Change Issue Reporter URL to point to our Mozilla PaaS instance
Merge pull request #945 from tschneidereit/issues

Adds a "Report Problems" entry to the extension's context menu