Fix for race condition and support for --net=host services.

* Fixed race condition in registrator.go causing some services to not be registered. This could happen when containers are starting concurrently with registrator, e.g. when using fig.
* Added discovery for services running with --net=host. For example Apache Mesos needs to be started with --net=host. This enables use of registrator for service discovery of Mesos masters which is very useful when posting tasks to Mesos from any cluster node. 
* Build and package registrator binary from checkout rather than downloading release from github. This eases development/testing of a registrator checkout.
* Error logging when storage backends raises errors. This shows what's happening rather than silently retrying/hanging when storage backends misbehave.