Allow timedelta parameters for expire, setex and psetex calls.
Removed call to datetime's total_seconds() in favour of a utility
function for compatibility with Python 2.6.
Added tests for expiry using timedeltas.
Merge branch 'ghaering-master'

* ghaering-master:
  Added tests for expiry using timedeltas.
  Removed call to datetime's total_seconds() in favour of a utility function for compatibility with Python 2.6.
  Allow timedelta parameters for expire, setex and psetex calls.