ENH: Added median_absolute_error to metrics
DOC: Added doc in model_evaluation.rst
ENH: unit tests for median_absolute_error
ENH: test_common.py unit tests pass
FIX: Small typo
Merge branch 'master' into median_absolute_error

Conflicts:
	doc/modules/model_evaluation.rst
Merge pull request #3761 from FlorianWilhelm/median_absolute_error

add median absolute error (MAE) regression metric