Use the 'itself' flag for 'respondTo' assertion.

This will check that the CommandMapper constructor itself responds
to the static method 'map' instead of the method on its prototype.
CommandMapper should be able to load JSON files as well.

I've augmented CommandMapper.map to be able to accept a file name
as well as an mapping options object. I've also added a new function
on the CommandMapper ctor called fromMappingJSONFile that will load
a CommandMapper object from a mapping.json file.