Improve performance when introspecting model classes

[SphtKr]

Using the utility class from #22, enable caching of model properties and similar information we're currently computing every time we serialize a given class. This should take a significant chunk out of processing time.

[SphtKr]

E.g. here: https://github.com/SphtKr/JSONAPI.NET/blob/master/JSONAPI/Json/JsonApiFormatter.cs#L554