issue #6: Reduce timeout while waiting for pong.

As every write causes snap to reset the connection timeout we want to
ensure that our own pings are not preventing an eventual timeout if
clients do not send back pong messages. To achieve this we lower the
timeout while waiting for a pong.
issue #6: Reset timeout on incoming data.

Even if no pong has been received, the fact that we read other incoming
data is taken as evidence that the connection is healthy and thus we
reset the timeout.
Merge pull request #7 from twittner/issue-6

Issue 6