Swipe recognizer using overall gesture direction and velocity instead of last interval

Related to issue #640. Swipe recogniser should depend on overall direction (offsetDirection) and overall velocity (overallVelocity) instead of the ones of the last interval. Makes this more sensitive to swipes that slow down before touchend.

Added "overallVelocity" property in input which takes into account the total gesture time and the overall distance moved. 

Prerequisite to fix issue #639 where the inverse value was returned for offsetDirection, also fixed in this PR.