added support for custom learners
fixes to support rescaling with custom regressors
Merge branch 'develop' into feature/custom_learners

Conflicts:
	skll/experiments.py
	skll/learner.py
Little clean up things for learner.py
added unit tests for custom learners.  strangely, the logistic one does not pass yet...
added specific fixed parameters and parameter grids to custom logistic regression test configuration
simplified tests/configs/test_logistic_custom_learner.template.cfg
Merge pull request #183 from EducationalTestingService/feature/custom_learners

Add support for custom learners