Make main module 'executable'

This is a change to make the main module of wavebender "executable" so that you can run `python -m wavebender` to get access to the command line tool, e.g.:

    $ python -m wavebender
    usage: wavebender [-h] [-c CHANNELS] [-b {16}] [-r RATE] [-t TIME]
                     [-a AMPLITUDE] [-f FREQUENCY]
                     filename
    wavebender: error: too few arguments