paver extensions

* adds a paver.ext namespace for extensions
    * mostly copies over code from mitsuhiko/flask for setting up the namespace (thanks, bro.)
        * implements a very rough thought on a convention for the cli to be able to invoke an extension's command
            $ paver <ext_name>:<task_name>
        * had to tweak the _import_task a bit to accomplish this
    * renames the project to avoid collisions with pip installs

-----

* adds a 4th positional arg to the cmdopts tuple, the "default" arg
    * sorry this mixes the commit, but i couldn't fucking stand it anymore
    * i wasn't around for the original line of thinking, so all i know is it was frustrating to not have something so simple