Update exportRecords.R

This is a great package and looks like it will be very useful for me and my team.  As I was working with it, I came across one issue.  Your exportRecords functions assume that the REDCap database has a 'study_id' field.  However, it is possible in REDCap to change the name of this field.  For example, in the database I am experimenting with, the field has been renamed to 'mrn'.  As a result, I could not pull select fields unless I removed 'study_id' as a default field.  

I do like the idea of always pulling the id variable, but there ought to be flexibility in what this field is.  I would recommend either not forcing 'study_id' to be pulled, or to add an additional argument, id = 'study_id'  At least this way I could change what my id variable is.
Merge pull request #1 from nutterb/patch-2

Update exportRecords.R