some cython stuff can't be processed by sphinx.
some platform-dependant modules can't get imported.
also: reordered modules in api.rst so order makes some sense.
the generation of those files was causing us way too much pain to
justify automatically generating them all the time.
those will have to be re-generated with `build_api` or `build_usage`
as appropriate, for example when function signatures or commandline
flags change.
see #384