Allow outgoing Subscriptions that expire in under 3600 seconds

This addresses:

  https://groups.google.com/forum/#!msg/sip_js/TshLqnjmjKY/uMXb6aihu3oJ

Note that RFC 6665 does not require subscriptions to last an hour, it merely
allows notifiers to reject subscriptions that are shorter:

  http://tools.ietf.org/html/rfc6665#page-18
Merge pull request #86 from joseph-onsip/shortSubscription

Allow outgoing Subscriptions that expire in under 3600 seconds