pxpcap: Allow compiling without bytearray support

When compiled with NO_BYTEARRAYS, the C++ extension will simply ignore
the bytearray setting and not use the new bytearray C API.  This option
should (with a little tweaking to the setup.py) allow compiling under
old versions of CPython, and (more significantly) allows compiling it
under the current version of PyPy.  Usage with PyPy hasn't been tested
extensively, but works at least well enough to run 'pxshark' on my Mac.
No word on performance yet. You have to copy the resulting shared object
into the appropriate destination yourself (the batch file/shell script
won't do it for you).