add support for `req.headers_in`, `req.headers_out`
separate execution of callable from `__call__`
refine support for `req.get_basic_auth_pw()`
add WSGI compliant wrapper for Basic auth

Add `wrap.BasicAuthModPythonWSGIApp`, a WSGI compliant wrapper for
a mod_python handler callable and a mod_python authentication handler

    application = BasicAuthModPythonWSGIApp(handler, authenhandler)