Export dialog (+bug fix)

Implemented export dialog where user selects what to output (single
study or entire database) and specifies the output filename; bug fix:
database helper reference to the study table is now correct
Dialog Cleanup

Worked out extending AlertDialog rather than a static method to get from
a builder.
Implemented export functionality

writes database to a spreadsheet (.csv) for either the whole data set or
for a single study (as specified by the user); augmented log for async
task status