Added a firefox search plugin

I just finished a plugin for Firefox, so you can now add SearchMash to your search toolbar, just like the standard Yahoo, Google and EBay ones. To install it, go to http://mashproxy.com/search/previewframe.html, and click on the "Add SearchMash to Firefox's toolbar" link.

Once it's there, you can do searches with SearchMash by clicking on the top right toolbar, that shows a 'G' and does Google searches by default, and select 'S' for SearchMash instead.