Bacon.sampledBy(observables, samplers, f)

I started to work on `Bacon.sampledBy(observables, samplers, f)` we have spoken of in https://github.com/baconjs/bacon.js/issues/383

It not ready yet. I'll add more tests and docs (By the way, as my english skills are very low, it a little hard for me to write docs. So if someone could do it instead of me, it would be very nice).

Just opened this PR so anyone can see what I am doing :)