[SPARK-5902] [ml] Made PipelineStage.transformSchema public instead of private to ml

For users to implement their own PipelineStages, we need to make PipelineStage.transformSchema be public instead of private to ml.  This would be nice to include in Spark 1.3

CC: mengxr

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #4682 from jkbradley/SPARK-5902 and squashes the following commits:

6f02357 [Joseph K. Bradley] Made transformSchema public
0e6d0a0 [Joseph K. Bradley] made implementations of transformSchema protected as well
fdaf26a [Joseph K. Bradley] Made PipelineStage.transformSchema protected instead of private[ml]