Added startTime( float ) option to Tween::Options.

I often found myself working around not having this built-in, so
decided to simply add it. Very useful when queueing a series of tweens
in one go that you don't have strictly starting/stopping one after
the other.
Merge pull request #286 from sansumbrella/timeline_additions

Added startTime( float ) option to Tween::Options.