parameterise the implementation to work with NSDictionary or Dictionary

[samdeane]

The current implementation is largely based on NSDictionary (as it's derived from the NSJSONCoding code).

It would be good to be able to build a version that just uses Dictionary, in order to:

• build on Linux
• be more efficient if we're just living in a swift-only universe

That said, it would still be more efficient to be able to use something like the current code when reading from / writing to NSDictionary.

One way that this might be achieved is to restructure the code to take a generic dictionary type, and then to provide boxing/unboxing implementations for both cases.