Support for Ingestion of Common JSON

There is an increasing demand to ingesting common JSON into the avro-encoded systems.

This release adds support for accepting common JSON , so we can take JSON from regular (non-avro-enabled) applications and encode it into avro per a schema in the expected way. This was a problem because union-types were expected to be of a form that satisfies the avro json encoding spec:

avro spec: ["null", "string"]
common json: null, or "some string"
avro json encoded: null and {"string": "some string"}

Example code, and tests are included in union_test.go