work within virtualenvs on Ubuntu

Unlike #72, which is about the structure of the virtualenv's metadata, ntfy doesn't work on Ubuntu because it needs to `import dbus`, and you can't `pip install dbus` easily, so you need to use the system version.

One potential fix for this is to simply hack in a `sys.path.append` that adds the directory where the importable `dbus` is found, import it, and then `sys.path.pop()` to remove it; this will be more restrictive than a full-on `--system-site-packages` virtualenv, but can still get that one library from the system.