A new RPC Routine to get the structure of a survey, and modify export_responses to return SGQA

I have added a new RPC routine "get_survey_fieldmap" out of the need to have some sort of reference to the results returned in the "export_responses" RPC routine.

I have modified the "export_responses" RPC routine so that it will return the SGQA that is used a field identifier. 

These modifications together will allow third-parties, such as myself, to build a mapping layer between LimeSurvey and another application.