Library requires Python 2.6 or higher
Include assert.h after Python.h

Fixes redefinition of _POSIX_C_SOURCE macro.
Allow any buffer compatible object as argument

This enables use of read-write buffers instead of only read-only buffers
as argument to the feed function. Also take additional offset and length
arguments so the caller can specify any slice within the buffer.

Thanks to @tzickel for contributing an initial patch (see #22).