Aa-stripe

Added
- creating subscription plans
- subscription support
- webhooks support
- added stripe_response field to all models to store RAW stripe response data
Changes
- customer_id on StripeToken to stripe_customer_id
- remaned StripeToken to StripeCustomer
- remaned all X.token to X.customer
- changed API url from /aa-stripe/tokens to /aa-stripe/customers
- moved creating stripe customer (on stripe) into instance method, not serializer.create. This way it can be in example used by cron.
- moved ``get_latest_active_customer_for_user`` into StripeCustomer as classmethod

Added
- related names to user model "stripe_tokens", "stripe_charges"
- utils.py with get_latest_active_token_for_user
- API to create StripeToken (Customer) out of stripe.js card authentication response. (uses Django-REST-Framework)
Changes
- renamed StripeToken.content to StripeToken.stripe_js_response

Added
- charging via StripeCharge model
- StripeToken model
- charge_stripe management command