Revised file processing screens on the backside of miniLock

Here is an implementation of the human interface design mentioned in issue #42.

And 3 videos that show it off:

Alice encrypts passport to Bob
https://vimeo.com/101773171

Save a secret song for my parents
https://vimeo.com/101773412

Crazy Town
https://vimeo.com/101773420
I’m not relying on the `error` boolean set durring postMessage because those messages don’t include the error object. The UI wants to know about the error message so that it can explain the reason for the failure. In order to get the error message I used a worker `onerror` handler so that the UI could display a simplified version of the message (which isn’t included on most postMessage errors). I did it this way because it demonstrated the human interface without making big changes to the worker.

Ideally, I think the UI error callback wants to know two things
1. What kind of operation failed? decrypt/encrypt
2. What is the reason for the failure? error.message

There are a few ways to satisfy that. I’ll push some changes to #55 that demonstrate what I have in mind.