fixes #136: implements software_u2f

[mozkeeler]

This is definitely a work in progress. I guess the kind of feedback I'm looking for is broad strokes on if this is the approach we want to take. Also, am I interpreting various arguments/return values correctly? Additionally, is there a way to test this end-to-end? (e.g. how do I configure the library to use the softoken instead of looking for a hardware token?)