Export custom fields with the ticket queue export

Also add in the user email address after the user name.

This patch also adds an ::export() method to the FormField() class which
allows a field to define how the data should be formatted when exported.
Merge pull request #396 from greezybacon/feature/custom-data-export

Export custom fields with the ticket queue export

Reviewed-By: Peter Rotich <peter@osticket.com>