Add support for reading Avro from RDD

[adamblev]

Currently it is only possible to parse avro from a file - spark.read.avro(filepath). We want to be able to parse avro from an RDD since our data is in a nested format. Currently this is supported with json format - spark.read.json(RDD[String]), we would want similar functionality for Avro but with binary.

If its possible to do this now with some other mechanism, would love to hear about it too :)

[cbyn]

#216