Experimental Python binding using ctypes

The current Python binding uses swig to wrap the C++ interface. This causes
problems on Windows, since compiling all of the DLLs correctly to work with
the various python binaries is painful, and makes the binding very large.

The idea here is to use ctypes to make a binding that's just Python and which
directly wraps the C API. The new vips8 C API is much more wrapper-friendly,
plus we can reuse parts of the gobject binding, so this should be possible. 


