CDAP-772 Feature/cdap 772 namespace service

Namespaces APIs to manage namespaces metadata.
Question: 
Should we create and use ``Id.Namespace`` instead of using namespace name as ``Key``?

- [x] Namespace service to create, list, delete namespaces
- [x] Unit testing, standalone testing
- [ ] Testing on a distributed cluster. 
I have a TODO for this :). I wasn't sure if it should be in cdap-api or cdap-proto. I see similar classes like ``StreamSpecification``, ``DatasetSpecification`` etc in cdap-api, while others like ``DatasetMeta``, etc in cdap-proto. What is the convention?