Possible bug in ComputeDCG(sorted_ranks)

Hi~
Thanks for sharing the code and the documentation. 
When I used your code to test the Duet model, I found that the function **ComputeDCG(sorted_ranks)** may have a bug on  the computation of DCG ( this code: **dcg += ((2^rating - 1)/math.log2(pos + 2))** ). 
In python, **^** means XOR symbol instead of exponent symbol. 

Kind Regards,
Linzhu