Add skopt.benchmarks functions

[glouppe]

For now, we could be move in the two benchmarks functions defined in the tests of gp_opt.py.

[glouppe]

Other possible benchmarks, as collected by @betatim in #2

http://automl.org/benchmarks.html
http://www.sfu.ca/~ssurjano/optimization.html

[MechCoder]

I really think, we should reopen this.

[AlexanderFabisch]

This might also be interesting: http://coco.gforge.inria.fr/

[glouppe]

@MechCoder What else do you have in mind for this? (I think we should avoid vague issues that will never be 100% fulfilled.)

[MechCoder]

OK, let us close this and add I'll add more specific ones.