added unique id to the service registration

Hi,

I'm trying docksul in a little project similar to Dokku (bash "intensive", tries to be very minimal) that uses consul to make a simple distributed PaaS. I wanted to have multiple instances of the same container in a host and because of that, the Consul API requires a unique ID for each service - since the name's been duplicated in the host. I used the container's ID to get that unicity during registration/deregistration and it works great. Thought you might want to include it.

I appreciate the many contributions you've made to open source, I've learnt a lot looking at several of your projects, so thanks!

David