[MCC-105863] RepresentorBuilder documentation

This PR:

* Adds xmldoc info for ```IRepresentorBuilder```. The docs aren't added to the concrete class because consuming apps should be coding against the interface, not the concrete implementation. Intellisense will show when coding against ```IRepresentorBuilder```.
* Added representor_builders.md doc with sample code
* Added ISerializer.ContentType with tests to match the work done during my spike
* Updated nuget package version to 0.1.4-alpha (0.1.3-alpha was pushed during my spike for testing)
* Included XML docs in nuget package for Intellisense support

@mdsol/hypermedia-team Please merge ASAP.