async for heavy calculations

[niklasfemerstrand]

Perform heavy calculations asynchronously so the browser doesn't freeze. HTML5 web workers don't have access to the window object so that won't work -- OpenPGP.js requires access to the window.crypto obj.

[niklasfemerstrand]

The OpenPGP.js project is already working on this individually, see: openpgpjs/openpgpjs#39

Can either work around their work on it or write patches directly for the library instead of for the plugin. Make a decision on this once it's critical, right now I think we're doing OK.