Add no-expire flag to not auto evict records on ttl

There is a scaling issue with ttl and heartbeats.  In the case of
docker you can have 1000s of containers running and with a lower ttl in
a dymanic environment it can become very chatty updating ttls for all
the records.  In the case of docker and skydock we know when a service
is removed so there is no need for the overhead of heartbeats because
docker will take care of that for us.  Also there is a race trying to
update 1000s of records that the heartbeat does not hit the cluster
before the ttl is automatically expired.
Allow records to be presistant based on NoExpire field
Merge pull request #84 from crosbymichael/no-expire

Add no-expire flag to not auto evict records on ttl