XEP-0198: Support "resend_on_timeout: if_offline"

If "resend_on_timeout" is set to "if_offline", resend unacknowledged
stanzas only if no other resource is online when the session times out.
In other words, allow for sending them to offline storage, but nowhere
else.
Merge pull request #300 from weiss/resend-if-offline

XEP-0198: Support "resend_on_timeout: if_offline"