create smsbillables app on hq
updates to the sms billable models for clearer understanding of usage and better distinction between types of fees
add backend instance to SmsFeeCriteria in smsbillables
Creation of accounting app in corehq, adding to settings

Conflicts:
	settings.py
phonenumbers returns country code as an integer, reflect appropriately in SmsGatewayFeeCriteria
ALWAYS store currency as DecimalField, not FloatField or you’re setting yourself up for trouble in the future
usage_fee and gateway_fee can be None on SMSBillable
api_response can be null as well, “” should be different from None. perhaps the API sends nothing unless there’s an error
initial sketch for creating an SmsBillable based on a message log
initial structure for tests on gateway fees
unhook Currency from accounting for now...
upped max_digits on currency containing decimal fields to 10 for inflated currencies
add HQAccountingSetupError exception to accounting models
add currency to accounting models
Revert "unhook Currency from accounting for now..."

This reverts commit 032fa7a1e78665521e578e20bd372cf814bd32ab.

Conflicts:
	corehq/apps/smsbillables/models.py
add blank=True to api_response in smsbilalble
move exceptions in accounting.models into accounting.exceptions
unlink accounting generator. create currency from Default, not specifically USD
save conversion rate from currency at the time the billable was created, make sure we catch 0.0 conversion rates
rate_to_usd should be rate_to_default for Currency