Add fake root to RST-dep tree

It is common practice in dependency parsing to add an artificial root node.
This root node governs the (actual) root(s) of the dependency structure.
It is usually placed at the beginning of the input sequence.

This artificial root node cannot have incoming dependencies, which should solve root-related downstream issues, e.g. in attelo's MST decoder.

I would welcome a switch (the code currently attempts to support Python3 in its half-arsed way), and it seems like all our dependencies now support it.

One bottleneck is attelo, which presently depends on Orange, which presently is Python 2.7 only.  If Orange goes Python 3, or if we manage to déorangiser attelo, we'd be golden.