Implement fully recursive file globing.

Python's base glob.iglob does not operate as if globstar were in effect. To
explain, let's say I have an erlang application with lager logs to

    /var/log/erl_app/lags.log
    /var/log/erl_app/console/YEAR_MONTH_DAY.log

and webmachine logs to

    /var/log/erl_app/webmachine/access/YEAR_MONTH_DAY.log

Prior to this commit, when configured with the path `/var/log/**/*.log` all
webmachine logs would be ignored by beaver. This is no longer the case, to an
arbitrary depth.

Signed-off-by: Brian L. Troutwine <brian@troutwine.us>