User class / resending of confirm link updated:

User class / resending of confirm link updated:
- If the provided email address for resending confirmation link is missing the
  filter will now redirect to a configurable URL
- New exception InvalidInterfaceException which will be thrown if a required
  interface is not implemented
- Resend link does now create a new hash and updates the user table
- Method BaseUser::ifEmailAddressExists() does now fix empty username attribute
- New method User::updateDatabaseField() added which updates database table
  columns with specified values
- Minor fixes applied