added u_law and a_law plugins

Signed-off-by: Bart Massey <bart@bart-massey.com>

This adds a couple of plugins useful in getting "telephone voice" sound effects. The plugin xml also works fine with swh-plugins-0.4.15, so you may want to incorporate these into the next release of your old LADSPA plugins as well.

Please don't hesitate to contact me if you have questions or concerns. Thanks much for all your awesome work on swh-plugins.

    Bart Massey
    bart.massey@gmail.com