Check whether the choosen encoding requires byte swapping

Check whether the choosen encoding requires byte swapping
for this iconv() implementation in the init function.

For encoding: use a byteswapped version of the input if
neccessary.

For decoding: byteswap every piece returned by iconv()
if neccessary (but not those pieces returned from the
callback)

Comment out test_sane() in the test script, because
whether this works depends on whether byte swapping
is neccessary or not (an on Py_UNICODE_SIZE)