Add toString to most datatypes

[nickrobson]

This adds toString methods (using Lombok's @ToString annotation) to most datatypes. I didn't add it to user- and payment-related types, as I don't believe they should have toString() methods because they contain PII. I'll leave that up to you, @mkotb and @aaomidi .

[aaomidi]

As mentioned in the chat, there are three issues with not including toString on these classes.

• The job of the API wrapper is to provide access to all data the API provides.
• Taking responsibility in hiding personally identifying information could make us liable in anyone expecting us to provide for "privacy".
• This project is essentially being developed from people in multiple continents and countries, with vastly different definitions of what constitutes "PII" and our responsibilities with them.