Fix utcoffset

[regnarock]

Usage of tzinfo must have been forgotten when coding this method. It works properly now.

$> python

>>> import pendulum
>>> tz = pendulum('Europe/Paris')
>>> tz.utcoffset(pendulum.utcnow())
datetime.timedelta(0, 3600)

[sdispater]

Thanks!

[regnarock]

Fixes #65