Payment functionality (integration with Stripe)

This version is fully integrated with Stripe.

Payment support

• Fully stripe integration (plans list is automatically generated from your Stripe account)
• User plans support
• Payments method support (only credit cards for now, by Stripe)
• Users can select a plan, change it, cancel, pause, resume
• User can see all the history of payment-related actions
• As soon as user logs in, the trial is started automatically (that plan that is marked in Stripe default one)

Dev's features

• All features are now divided to units and components. Frontend and backend are put side-by-side for easier reference and development.
• Integration with Google App Engine (reading entities if env variables are not accessible)

Small but pretty user friendly features

• Loaders to show user when data is fetching but still not finished
• Loaders may be easily added to buttons