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
Why have you removed Math.ceil() here? There is a legitimate reason for its inclusion: it produces more conservative estimates.