Added 2 new options for forcing direction of the dropdown 

Hello,
I added simple tweak to force direction of the dropdown either bellow or above the input. The new options are

forceOpenBelow and forceOpenAbove

This was an issue discussed here:

https://groups.google.com/forum/#!topic/select2/-1GWUw3q_JA

or here:

http://stackoverflow.com/questions/19983601/prevent-select2-from-flipping-the-dropdown-upward