Add support for exportFieldNames call

[chgreer]

Sometime after this project stopped updating, REDCap added a call to export a list of field names via the API. This pull request adds this into PyCap.

In particular, the API has different behavior around *_form_completed fields. These fields are not present in the metadata export used by PyCap to determine the field names for a project. At the same time, the export_field_names fields don't include some field names in the metadata. As such, I simply implement this method and let the user decide what they want to do with it.

import redcap

project = redcap.Project(<URL>, <TOKENHERE>)
project.export_field_names()

Returns a list of dicts like:

[{'original_field_name': 'participant_id',
  'choice_value': '',
  'export_field_name': 'participant_id'},
 {'original_field_name': 'survey_respondent',
  'choice_value': '',
  'export_field_name': 'survey_respondent'}]

[pwildenhain]

Thanks for adding this in! Just a few small suggestions

[chgreer]

Ah. I just clicked 'accept suggestion'. I've never used this feature of github. So hopefully that worked. Let me know if it didn't take.