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)
Actually, we should make the call to create_billable_for_sms go right before this function ends, because as it is right now we're missing out on billing verification or registration sms.  If we want to make sure it matched a domain, we can just make the last two lines of this function be if msg.domain: create_billable_for_sms(msg).