Removed instrument context from response set. #2668

The design for the feature has changed. Now, the mustache context will be generated from the response set. See #2668.
Designed response set mustache context interface. #2668
Generate dictionary of responses to mustache helper questions. #2668
Testing NUQuestion reference identifer helper methods. #2668
Giving ictionary of mustache context to nu_surveyor. #2668
Method signature change. #2668
Merge pull request #2 from NUBIC/2668-mustache

Add mustache support in surveys