Fix step rate bug (especially for retraction)

This is the fix for the issue discussed here:

http://umforum.ultimaker.com/index.php?/topic/1734-marlin-bug-the-myth-of-retraction-speed/

It mostly affects fast retraction moves, but could affect any axis with the right combination of speeds and accelerations.

(This is my first github pull request... let me know if I'm not doing it right...)