CRUNCH-433 Add support for AvroKeyValue file

Add support for reading AvroKeyValue files created using
org.apache.avro.mapreduce.AvroJob. Also add explicit methods in
the From class for reading an Avro key/value file as a PTable.

Signed-off-by: Josh Wills <jwills@apache.org>
CRUNCH-439: classifier for avro-mapred