Billing domain filter fix

Fixes a **PRIORITY 2** Billing Bug: 
http://manage.dimagi.com/default.asp?132430

adds:
- regression test so this gets caught in the future
- fix to repair old `SMSBillables` with unspecified domains.
- performance improvement to `DomainFilter` in the billables report (yeah, this could have been a separate PR :P)
So we don't bill for inbound SMS that isn't handled? is that by design?