Add setting to prevent easing being reversed when playing in reverse

When going backwards you don't always want the easing reversed. This commit adds a new setting to skip the automatic easing reversal when playing in reverse. 

For example I'm making a simple slider with a transition forward of ease-out. When reversing I still want the transition to be ease-out. Without this patch the easing gets reversed automatically so it looks like an ease-in when going backwards and an ease-out when going forwards.

Example fiddle : http://jsfiddle.net/JrJsz/