Add yaml_hook to GetValue
Switch MultipleChoice 'correctanswer' to 'answer'

Close #19
Add yaml_hooks to more question types

This should point us a bit more toward completing #25.
Renamed ExperimentalQuestion to RecordingQuestion and made it abstract.
Merge branch 'master' into f.aggregate_shellq
Renamed Experimental.py to Recording.py. Made minor changes to PrintingQuestion.