Make Requests/Responses strongly typed classes

[iNetDaniel]

Working with anonymous types in C# is a pain to deal with. There are limitations like the properties being read-only after initialization, not being able to have re-usable functions that convert from our models to TaxJar's, not having access to intelli-sense outside of the function the objects were initialized in, etc.

It would also be nice if we could reference API documentation inside of the IDE object explorer via summaries.