EXP-58. Allow settings on ModelDefinition to be modified.

This commit modified ModelDefinition to allow users to override
settings on a model definition.

Reviewed by: robertchu
Adds pack/unpack methods to KijiPipe for AvroRecords.

Reviewed by: kiyan
EXP-59. Allow settings on ModelEnvironment to be modified.

This commit modified ModelEnvironment to allow users to override
settings on a model environment.

Reviewed by: robertchu