* Instead of check.n_pings (int) use last_ping().n
* Instead of check.last_ping (datetime) use last_ping().created
There is a time gap from creating a flip object to processing
it (sending out an alert). We want the notification to reflect
the check's state at the moment the flip was created. To do this,
we use the Transport.last_ping() helper method which retrieves
the last ping *that is not newer than the flip*.
This commit updates transport classes and templates to use
Transport.last_ping() consistently everywhere.
