enroll-stripe-coupons: Add coupons django app to handle coupons & pay

Request payment upon enrollment: The feature is activated by adding a
course mode `pay_with_coupon` to a course.

When activated, when a student attempts to enroll on a given course
session, it gets to apply a coupon reduction if he has one. A Stripe
payment form is presented to him, and he only gets to actually enroll
once the payment is cleared. Staff is unaffected. The coupon code is
stored with its enrollment.

Also allow to configure a different price on each course, using the
`CourseMode.min_price` attribute.

Applying the `pay_with_coupon` mode to a course:

```
from course_modes.models import CourseMode

CourseMode.objects.get_or_create(
    mode_slug='pay_with_coupon',
    mode_display_name='Pay with coupon',
    course_id='CodeCoalition/CC-101/test',
    min_price=125)
```

Creating a coupon:

```
from coupons.models import Coupon

c=Coupon.objects.get_or_create(price_reduction=30, coupon_name='edX')
```

Configuration parameters:

* `STRIPE_KEY_PUBLIC` and `STRIPE_KEY_SECRET`: Stripe API keys