Fiy python2.7'ism (#566)

Fiy python2.7'ism (#566)

The oauth plugin used timedelta.total_seconds which was introduced
in python 2.7 only. To preserve backwards compatability, we simply
calculate the time difference in seconds manually.

I considered monkeypatching total_seconds to the timedelta object,
but it is a built-in type written in C (I believe) and modifying
attributes failed horribly. Switch this to use total_seconds once we
require python 2.7 as minimum version.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>