0.1.0 - first release
0.2.0 - static analysis
0.3.0 - dispatch on instance methods
0.4.0 - support for namespaces
        register methods a la singledispatch 
        documentation
        Faster caching performance
0.4.1 - expose ambiguity response via callback
        Improve NotImplemented error messages to include missing signature
0.4.2 - Dispatcher is serializable
0.4.3 - Dispatcher.register handles union types
        (previously only @dispatch covered this)
0.4.4 - Docstrings
        Better error handling on bad type inputs
0.4.5 - halt_reordering function to stop reordering after each declaration
0.4.6 - Rename Dispatcher.resolve to Dispatcher.dispatch to adhere to
        singledispatch api.  Also change to variadic args
        .dispatch returns None rather than raising an error on null result
0.4.7 - MDNotImplementedError allows Dispatchers to continue on to the
        next-most-specific implementation