EXP-23. Implement a Scala wrapper around KeyValueStores

This commit introduces the KijiExpress KeyValueStore, to be used
as part of the modeling SPI. A KijiExpress key-value store is a
Scala-friendly face on an existing KijiSchema key-value store, that
handles converting key/value types appropriately for users.

Reviewed by: robertchu
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