Renamed go-gsl -> gsl.
Merged all packages involving rngs into package random.

Per discussion on golang-nuts

<https://groups.google.com/forum/#!topic/golang-nuts/OxLaNEogixY>

it is not easily possible to pass custom C types between packages
without some serious pointer gymnastics.
Thus, I combined all packages which require random numbers into
a single random package (rngs, qrngs, random distributions, ...).