Add streaming analytics doc page

This will probably become a blogpost (though not until the work is actually in to the various `*toolz` projects.)

@eigenhombre The join PR benefited significantly from your comments.  Your perspective would be appreciated here.
I think the first sentence can be improved.  Perhaps change the final clause to "not a library *specifically* for data analytics", because `toolz`, while general purpose, can certainly be used for data analytics.