New option in rands_config.h

Now it is possible to remove stdlib's rand from the
speed test
First experiments on Xorshift with SSE

The results show that the commented version
is twice times slower than the current one.

Compiling to assembler shows that the SSE version
uses 3 orders: 2 moves (movdqa) and a shift (psrldq),
and the other version 5 moves (movl).
New version of Mersenne Twister

The loops inside the mt_calc() function have
been unrolled, this avoids the calculation of
'%624' thus speeding up the algorithm nearly
by a 40% (as measured by the speed test)