EXP-48. Allows dynamic creation of AvroRecords for writing to Kiji.

Also cleans up AvroUtil.encodeToJava to generate schemas dynamically
instead of a schema argument.

Reviewed by: robertchu
EXP-42. Adds KeyValueStores trait.

KeyValueStores provides access to kvstores to the various stages of
a model spec.

Reviewed by: kiyan