ResourceDictField behaves sort of like a standard dict, but it

wasn't one. It lacked a lot of the functions and behavior that a dict

would support. It wasn't mutable, and didn't pretend to be, but the lack

of standard mutable dictionary operations made it a bit confusing.
This change updates this class to inherit from MutableMapping, giving

it a standard dict interface. All mutable operations are now available

on the class, but will raise a helpful message explaining that the

dictionary cannot be modified and how to do an update to the resource,

which should be useful to people working with the API.
Subclasses will build upon this to offer better immutable guidance (for

the extra_data support).

Unit tests pass on all supported versions of Python.