International Support

[bpepple]

Is your feature request related to a problem?
To handle non-US based publishers, we need to add some international support.

Describe the solution you'd like
At a minimum, we should add the following:

• Add a country field to the Publisher model
• Modify the price field to be a list that can handle different currencies

Most likely we could use a library like PyCountry, django-countries, or django-money to simplify the code.

Additional context
Modifying the price field will be a API breaking change, so we should see what other changes we want to make, like adding Versioning to the API. This will also need to announced in advanced, so the various consumers are made aware.

[bpepple]

We should look at #181 when working on this bug.

[bpepple]

Added Country field to the Publisher model with #332