Add encryption to protect API keys

[alexandrfox]

At the moment JSON file with API keys is stored in plain-text format. I think it would be nice to offer optional (?) encryption to protect its contents.

[johntitus]

I just pushed a solution to this in 0.9.0. You'll need to upgrade to get the coinx lock and unlock functionality.

I'd appreciate as many eyes on this code as I can get, just to make sure I didn't do anything stupid. I basically decided to hash the file using a password. I enforce the password being minimally-strong.