Add icons to the extension

I downloaded the icons from [FreeAgent's SSB page][], manipulated the
512px icon in Acorn with help from @floehopper, created the smaller
sizes and specified them in the manifest.

I've kept the 512px version even though it's not actually used by the
extension.

[FreeAgent's SSB page]: http://www.freeagent.com/support/kb/misc/creating-a-site-specific-browser-for-freeagent