Move python/cython code out of underscored directories

Moved subsampling and SSW python/cython code out of directories that start with an underscore. nose is hardcoded to skip directories named in this way, so the doctests were not being run (only 1 doctest was actually being skipped). Put the actual SSW code (not the wrapper) under a `_lib` directory as there will never be unit tests or doctests for that code.

Fixes #658. @ebolyen can you please review?