Don't try to cast a python string in Image.from_buffer

Doing this just results in an error, and isn't necessary anyway. cffi accepts python strings as `void *` arguments.

The error I get, at least, is:

    >>> image.ffi.cast('char *', 'dongs')
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File ".../local/lib/python2.7/site-packages/cffi/api.py", line 233, in cast
	return self._backend.cast(cdecl, source)
    TypeError: an integer is required