README: fix import and reference of datetime functions
Merge pull request #46 from glarrain/patch-1

README: fix import and reference of datetime functions
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).
Update README.md
Merge pull request #41 from skion/nbf

Not-before claim validation
Add support for alg="none".
Verify method for alg="none" was redundant.
Remove redundant check for alg="none".
Fix test for new exception.