[Compiler] Add support for pluggable specification languages

[wwerner]

Currently, we only support our own specification language/DSL.
During the demo with the reactive foundation, the question for additional specification languages came up (JSON Schema was mentioned by Ryland).

I think we could fit it into the architecture quite nicely, since the DSL is decoupled from the generator and the validation by means of the AST.

If we also add a generator that spits out the Vlingo DSL, we could also implement versioning in a manner independently of the specification format by converting specs in other languages to the Vlingo DSL first and apply versioning to this normalized spec. But I haven't thought it through completly yet, I guess we need to take care not to break roundtrip workflows in this case

Vlingo Spec DSL ->			-> Java
JSON Schema ->				-> C#
Avro ->				AST		-> ...
Protobuf ->
					/ \
					 | 
				 Validation
				[Versioning]

[VaughnVernon]

@wwerner Include support for Avro and Protobuf definition languages.