Implement flow control for the incoming stream.

This sends a WINDOW_UPDATE frame whenever the total number of
unacknowledged bytes exceeds 32 KiB. This is a simple policy;
we could tune it to be more sophisticated as necessary.

This fixes downloads of streams greater than 64 KiB.
Merge pull request #84 from square/jwilson/send_window_update

Implement flow control for the incoming stream.