First stage of API re-work

This partially addresses #72.

Rename the ``dtcwt.backend.backend_{numpy,opencl}`` modules to
``dtcwt.{numpy,opencl}``. Remove ``dtcwt.backend.base`` since it was
implemented with NumPy anyway.

Make ``TransformDomainSignal``, ``Transform2d`` and ``ReconstructedSignal``
top-level members of ``dtcwt``. For the moment they are hard-wired to be the
NumPy implementations but a runtime switching interface will come soon.

Remove the needlessly complex NumPY registration module and move all functions
into ``dtcwt.registration``.

Move all of the MATLAB compatibility stuff out into its own ``dtcwt.compat``
module.

The very existence of ``ReconstructedSignal`` needs to be examined before #72
can be closed.