Added an option to have a preferred random number generator.

This 'preferred generator' is the one that will be used in some
tests and in the distribution generators that will be implemented
in a future.

The define must be a function that generates random numbers
(technically, it can be anything that can be assigned to an
unsigned int). So, any of the `*_rand()` will be enough.

There's a big problem with the approach I have taken for this,
mainly because I still don't know how to do it better. If
you choose not to use all the generatos and you choose as a
preferred generator one that isn't activated... well, let's
say that it won't work.
Some tiding of the code
Stop showing stupid caret information about errors
Started a new branch to add SSE support to Parisi
Added full code for SSE2 version of parisi

To be able of make more than one calculations at a time,
I use the trick of Isaac generator: keep two wheels, one
with the 'true' state of the generator, and another with
the future output. Thus, I can calculate in advance 256
random numbers making use of SSE and after that returning
them as they are needed.

Although this may seem simple, care must be taken with
the wheel's bounds and the positions from where we have
to combine numbers, as they are not multiple of 4.

This can be seen when I have to calculate 12 random numbers
one at a time instead of using SSE. This is because the
first three (52-54) can't be done with SSE, we could do
the next four (55-58) in one SSE step, but the next five
(59-63) have to be done one at a time.
Final changes for Parisi-SSE
Merge branch 'pr_rand_sse' into devel
Started a new branch to add Mersenne Twister
First version of Mersenne Twister

There's still work to do for fully integrate it
in the rest of the library. The next commit will
be this.
Final changes for Mersenne integration
Merge branch 'mersenne' into devel
Solved a bug in pr_sse
Merge branch 'devel'
Bumped to version 0.6