This is very nice!
While playing I did notice one thing though: the game seems to allow consecutive combinations when it shouldn't. To clarify, if one row is 4,2,2,_ and you press 'right', you end up with _,_,_,8 (it looks like it combines the 2s, then the resulting 4 with the other 4) instead of _,_,4,4 as it should be.
Anyways, great job, I've enjoyed this a lot!