move existing lowlevel code to explicit numpy backend
a first stab at moving functionality to a backend class

Move the lowlevel filtering code to a NumPy backend class
utils: remove confusign __all__ for the moment

Whereas __all__ can be a useful thing to have, while we're re-factoring
it can cause more pain than it is worth.
move 2d forward transform to backend implementation
fold dtwave{i,x}fm2b into main transform
refactor opencl backend to support modified rotational symmetry wavelets
move opencl backend to be where numpy backend is
fix some imports in the NumPy backend

Make sure that the NumPy backend uses its own colfilter, etc low-level
API instead of trying to use the indirect main API.
update documentation on backends
Merge pull request #31 from rjw57/backend-refactor

refactor backend support to actually separate functionality