[Codeplex taks #1776] Support Singleton feature

Singleton is the new concept in OData V4. The meaning of it is to allow
user to create a special (unique) entity and such entity can be addressed
directly by its name. The behaviour of Singleton is very similar with
the behaviour of entity set but has different meanings.

Here is the scope of singleton feature:

1. Support Singleton configuration
2. Support Singleton manipulation on ODataModelBuilder
3. Support Singleton manipulation on ODataConventionModelBuilder
4. Support Singleton convention routing & attribute routing
5. Support Singleton serialization / dieselization
6. Support Singleton query option
7. Support Singleton navigation property
8. Support Singleton in Service document
9. Support Singleton action and function