Refactor into class methods
Refactor renderers.JsonApiMixin.render

Iterates through wrapper methods in .wrappers, returning the first
successful one, or raises WrapperNotApplicable.  Users can change
functionality by altering the list of wrapper methods, adding their own
methods.
Use dict_class when creating dict-like objects

This allows the user to switch to another dict class, such as
OrderedDict or SortedDict.
Call util functions through class methods

This allows a user to override how models are derived from objects and
how models are converted to resource names.
Merge pull request #12 from jwhitlock/refactor_7.2

Refactor parser and renderer for extensibility