Add Schema Printer

This adds a printer to translate the result of an introspection query
to the grammar that is currently used as shorthand in the specification.
This should be treated as the de facto specification until a formalized
grammar is produced.

The next step is to do the transformation in the other
direction.

A tool that should be built on top of this is one that executes
the introspection query against an arbitrary GraphQL HTTP endpoint
(specified per the upcoming graphql-http spec) and then performs
acceptable tests with that tool.

In the interim we hope that this will be useful for implementors
building and debugging their own type systems.

Author: schrockn-zz