EXP-30. Allow users to run KijiSchema DDL Shell from KijiExpress Shell.

This commit adds a command to the kiji-express shell, :schema-shell,
which runs an instance of the KijiSchema DDL Shell within the REPL.
The README is also updated with information about the express shell.

Reviewed by: daisy
EXP-19. Adds Extractor trait for Model SPI.

Reviewed by: kiyan
EXP-21. Adds Scorer trait for Model SPI.

Reviewed by: kiyan
Modeling package.
Adds more tests to suite.