Added evaluateEachIteration to allow the user to manually extract the error for each iteration of GradientBoosting. The internal optimisation can be dealt with later.

Author: MechCoder <manojkumarsivaraj334@gmail.com>

Closes #4906 from MechCoder/spark-6025 and squashes the following commits:

67146ab [MechCoder] Minor
352001f [MechCoder] Minor
6e8aa10 [MechCoder] Made the following changes Used mapPartition instead of map Refactored computeError and unpersisted broadcast variables
bc99ac6 [MechCoder] Refactor the method and stuff
dbda033 [MechCoder] [SPARK-6025] Add helper method evaluateEachIteration to extract learning curve