Add `randomSample` transform

[Osmose]

trafficstars

S&I has a few usecases for random sampling, mainly being used in conjunction with a stable sample in order to spread out the distribution of the stable-sampled audience over time.

Transforms seem to be the way to go, so how about:

0.5|randomSample()

@rayborn @mythmon How does the syntax above look?

[mythmon]

LGTM

[mythmon]

Filed bug 1435883 for the client side of this.

[shell1]

Fairly complex to do on the client side - so holding on the server side work until client can be done. Check into the bug for progress. Not high priority - but on radar.