add absolute_import from __future__

Before re-naming the various backend implementation modules, it is worht
making sure that absolute imports are switched on.
rename dtcwt backend modules

A bit rename patch mapping ``dtcwt.backend.backend_numpy`` and
``dtcwt.backend.backend_opencl`` to ``dtcwt.numpy`` and
``dtcwt.opencl``.
move numpy registration implementation to top-level module

Until there is actually an OpenCL implementation, keep it simple,
stupid.
move MATLAB-style API into dtcwt.compat module

Strongly encourage use of new-style API.
update README with API change