Add not-before validation.
Update README.md

Add `nbf` claim mention.
Unit tests for leeway and skip verification.
Add py34 (tested OK on Ubuntu 14.04).
Merge branch 'nbf' into nacl-pure

* nbf:
  Add py34 (tested OK on Ubuntu 14.04).
  Unit tests for leeway and skip verification.
  Update README.md
  Add not-before validation.
Transparent base64 decoding upon key preparation.