Normalize listen durations, defang erroneously long listens.

Sometimes, listens are coming in with extremely long durations.

Basically, in the event that the browser has gone really wonky, we
treat it as a 15 second listen.

Might need to add a special case for negative values, which, while also unlikely,
are possible.

FIXES PRXNXT-507
Normalize negative listen durations, as well.

There's a (small) chance that some of the extremely high values are so
high because Google Analytics converts from a signed integer to an
unsigned integer, making a negative number into an extremely large positive
number.

This seeks to address that. I added a small window (of under 0.5 seconds)
because it felt right to do so. Not sure if it actually makes sense.
Merge pull request #86 from PRX/fix/long-listens

Fix very long listens