AWS ELB entity (i.e. load balancer)

Builds on @nakomis 's work for adding an ELB entity.

Note that it uses the aws-sdk, which adds a 10MB dependency! I've therefore put it in a separate mvn project, but it will still bulk out the .tar.gz dist. Unfortunately the ELB domain objects and web calls are not in jclouds; we could add those with a medium amount of effort.