Updated grid objective function validity test

Now allows function types 2,3 and 4 for regression models
And checks for class types in classification models
Adds a simple backoff in case the classes are of mixed type
Added tests for regression grid objective funcions

Needs to be extended for classification case
corrected 'type-2' list
Merge branch 'feature/fix-grid-objective-check' into feature/fix_unit_tests

Conflicts:
	skll/learner.py
	tests/test_skll.py