Implement low credit alerts in billing

Having the billing API alert the account holder when there credits drop below a specified threshold.

Currently the billing API marks this is marked as TODO at https://github.com/praekelt/vumi-go/blob/e4320d12c81bc122cc9b70f96492c59ee1a58290/go/billing/api.py#L755-L757
This won't work because ``go/billing/api.py`` runs under Twisted -- i.e. we need to use something like deferToThread (http://twistedmatrix.com/documents/9.0.0/api/twisted.internet.threads.deferToThread.html) to call this. If you like, I can try write an example and add it to this branch?