Fix issue #56 (infinite loop) in calculateDuration

* In PrettyTime.calculateDuration(), when the unit was englobing the
absoluteDifference, the delta was not correctly set. It should be set to
zero whenever it is englobing the absoluteDifference.
* Removed unused argument in getSign()
Merge pull request #67 from nguydavi/master

Fix issue #56 (infinite loop) in calculateDuration