EXP-132. Extend model definitions and environments to allow users to specify PMML to use for model.

issue: https://jira.kiji.org/browse/EXP-132
review: https://review.kiji.org/r/1058
EXP-194. Allow multiple inputs/outputs to a single phase of the model lifecycle

issue: https://jira.kiji.org/browse/EXP-194
review: https://review.kiji.org/r/1054
EXP-195. Add support for Text Input/Output sources in the Modeling Lifecycle.

issue: https://jira.kiji.org/browse/EXP-195
review: https://review.kiji.org/r/1063/
EXP-136. Implement a trainer for linear regression models.

issue: https://jira.kiji.org/browse/EXP-136
mostly done LR