Overhauls the transformer namespaces so that all Transformer instances can now be accessed directly from the .extractors, .filters, and .converters modules, respectively. A secondary benefit of listing all subclasses of each type in __all__ is that the get_transformer and get_converter methods are now much simpler, since we no longer have to recursively walk each transformer module tree.
improved namespace management
improved Converter namespace
fix get_transformer function and amend imports
simplified imports and minor fixes
Coverage decreased (-0.6%) to 78.168% when pulling 12bf426 on simplify-namespaces into 5dbba5a on master.