Support for Tiered Metered Usage Plans

[jksimoniii]

Issue Summary

./manage.py sync_plans throws an error when attempting to convert a plan that uses Stripe's Metered Billing feature (docs).

Steps to Reproduce

Setup a tiered metered usage plan with metered billing in Stripe (see docs) and then run ./manage.py sync_plans

What were you expecting to happen?

New plan gets synced to Django.

What actually happened?

Traceback (most recent call last):
  File "manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/usr/local/lib/python2.7/site-packages/django/core/management/__init__.py", line 364, in execute_from_command_line
    utility.execute()
  File "/usr/local/lib/python2.7/site-packages/django/core/management/__init__.py", line 356, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/usr/local/lib/python2.7/site-packages/django/core/management/base.py", line 283, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/usr/local/lib/python2.7/site-packages/django/core/management/base.py", line 330, in execute
    output = self.handle(*args, **options)
  File "/usr/local/lib/python2.7/site-packages/pinax/stripe/management/commands/sync_plans.py", line 11, in handle
    plans.sync_plans()
  File "/usr/local/lib/python2.7/site-packages/pinax/stripe/actions/plans.py", line 12, in sync_plans
    sync_plan(plan)
  File "/usr/local/lib/python2.7/site-packages/pinax/stripe/actions/plans.py", line 25, in sync_plan
    "amount": utils.convert_amount_for_db(plan["amount"], plan["currency"]),
  File "/usr/local/lib/python2.7/site-packages/pinax/stripe/utils.py", line 36, in convert_amount_for_db
    return (amount / decimal.Decimal("100")) if currency.lower() not in ZERO_DECIMAL_CURRENCIES else decimal.Decimal(amount)
TypeError: unsupported operand type(s) for /: 'NoneType' and 'Decimal'

Updates

• The error likely comes from tiered plans having amount: null

>>> stripe.Plan.retrieve("somestring")
<Plan plan id=somestring at 0x7f8136441178> JSON: {
  "active": true, 
  "aggregate_usage": "sum", 
  "amount": null, 
  "billing_scheme": "tiered", 
  "created": 1548879965, 
  "currency": "usd", 
  "id": "somestring", 
  "interval": "month", 
  "interval_count": 1, 
  "livemode": false, 
  "metadata": {}, 
  "name": "somestring", 
  "nickname": "somestring", 
  "object": "plan", 
  "product": "somestring", 
  "statement_descriptor": null, 
  "tiers": [
    {
      "amount": null, 
      "flat_amount": 14900, 
      "up_to": 100
    }, 
    {
      "amount": 100, 
      "flat_amount": null, 
      "up_to": null
    }
  ], 
  "tiers_mode": "graduated", 
  "transform_usage": null, 
  "trial_period_days": null, 
  "usage_type": "metered"
}