Add direct article links to MathSciNet

Rewrites 'article' links on MathSciNet pages to link directly to the PDF, if possible, rather than the publisher's web page.

There are also settings allowing you to:
* Display a copy of the PDF inline at the bottom of the MathSciNet page.
* Automatically download a copy of the PDF (optionally zip'd, to get around browser prompts).
* Save a copy of the PDF inside browser storage.
* Save a copy of the PDF to your Dropbox account.

If you save a copy to browser storage or your Dropbox account, later page loads will reuse this copy.

Finally, it is possible to configure the extension to perform these download or save actions on search result pages too.