[model][client] Custom metadata model registration

[changhiskhan]

To support type custom fields and richer metadata modeling, we implement the registerModels feature of the SDK to convert @MClass models into JSON to be registered with the server. We're using the new custom property feature in Navigator 2.6 to create managed custom properties that have a type and allow validation constraints to be specified. These properties belong to namespaces and are associated with various meta-classes (i.e., hdfs entities, Hive tables, etc).

Closes #52 Closes #53

TODO:

• [x] Fix hard coded namespace
• [x] Make API version an explicit config variable
• [x] Fix broken tests
• [x] Clean up examples
• [x] documentation