An object graph for tests

Closes #158:

  * Adds the package `com.microsoft.tang.test` in the `tests` folder that contains an object graph and configuration according to #158. 
  * There is a test of instantiation and equality in `ObjectTreeTest`. 
  * There is `RoundTripTest` that can be used to test serializers.
  * We now use that for the Avro serialization.