Changed references to encrypt/decrypt to mask/unmask.

This is in reference to https://github.com/justinas/nosurf/issues/11

Specifically:

* All references to "encrypt/encrypts/encrypted" have been changed to "mask/masks/masked"
* All references to "decrypt/decrypts/decrypted" have been changed to "unmask/unmasks/unmasked".

Although a one-time-pad is technically encryption, I think the change helps clarify what is actually taking place (unique-per-request tokens) and will remove any conflicts/confusion should you incorporate authenticated/encrypted cookies down the track.