Free xor circuit security bug

Fixed issue #27.

Denote the wires entering a gate as A and B, and denote the exit wire as C. Denote by T the gate number. All values are 128 bits long. 
Instead of computing  H(A+B+T) + (A+B+T) + C, (where + is the xor operation) now compute H( 2A+B/2+T) + (2A+B/2+T) + C.

Now the implementation is secure for free xor circuit also.