Add payments system

[pierd]

We should try to make some simple integration with Pin Payments.

As for the system there are some open questions:

• prepaid vs postpaid?
• are there cancellations and how do they work?
• are prices flat? tied to specialisation? totally customisable by therapists?
• are there different session types with different prices?
• disputes?

This is one big system on its own. The charging integration is actually the easy part.

[dav-armour]

There is a ruby gem to interact with api we can use to make it slightly less painful. https://github.com/dNitza/pin_up/