Use constant time string comparison routine from hmac module
Use constant time string comparison routine from hmac module

...and only use the current implementation as a fallback.

Fixes #63.

mark-adams/pyjwt,jmgamboa/pyjwt,Wolfium/pyjwt,rainierwolfcastle/pyjwt,progrium/pyjwt,jpadilla/pyjwt,mindw/pyjwt