Add $required_fields array for edd_purchase_form_validate_logged_in_user() function

Allows filtering of required fields on purchase form.
add edd_purchase_form_required_fields() function

- New edd_purchase_form_required_fields() function for controlling the required purchase form fields
- edd_purchase_form_validate_logged_in_user() function now loops through required fields function and displays error messages accordingly
Merge branch 'master' of git://github.com/sumobi/Easy-Digital-Downloads into sumobi-master