Migration to dbus-next (a fork of dbus-native) is a rewrite, but fixes
some outstanding bugs in the project related to variant types.

The project is now transpiled to use the experimental decorator feature
that will be available in the language at some later time. gulpfile.js
contains build instructions. Build with `npm run build`. The dist/
folder contains what will be published on npm.

Interfaces are implemented as classes with decorators specifying the
properties of the member that is exported on the bus.

Update examples and add a new tracklist example.

Other bugfixes may have been a side effect of the rewrite.

fixes dbusjs#1
fixes dbusjs#6
fixes dbusjs#13