un-complect wiping zeroes and wiping a slot

Yes, we iterate through the list twice, but we no longer have
a function that does two things.
wipe zeros *before* wiping slots, ensure that zeros get reported.

This commit is best explained by describing the behavior of
RollingTopWords in its absence.

Let's say the word "Bieber" appears in the tweet stream at a very high
rate for half an hour, and then is *never heard again*.

This will insert the word "Bieber" into our rankings object downstream.

Note that there is nothing to *remove* the word "Bieber" from the
rankings object downstream. Let us assume that, during its half-hour,
the word "Bieber" appears *more than five times as often* as other
highly-ranked words. Then the very *last* "Bieber" report (in which
"Bieber" has only appeared for the first minute of the five-minute
window) will still be able to hold a place in the rankings. Objects are
only lowered in the rankings by being outranked, or by *appearing again*
with a lower score.

*With* this change, we wipe zeros, and *then* wipe a slot. This means
that when we wipe the slot containing the last Bieber mentions, we
don't at that moment remove "Bieber" from our map. The *next* report
will include "Bieber" with a score of zero, removing "Bieber" from the
ranking object downstream.
Merge pull request #33 from MichaelBlume/emit-zeros

Ensure zeros get reported.