[C++-sig] Pyste default module names erroneously include path

[paul.bridger]

When I run pyste from a parent directory to the target pyste file, and don't 
specify the module name (eg. >pyste.py foo/bar.pyste), pyste calculates the 
module name incorrectly as foo/bar.

/* bar.cpp */
BOOST_PYTHON_MODULE(foo/bar)

I'm not sure if it matters, but this is done from a python build script:
os.system('pyste.py --cache-dir=cache --pyste-ns=__hidden %s' % (filename,))

Paul