HDNode: add optional network flag for importing

This should resolve https://github.com/bitcoinjs/bitcoinjs-lib/pull/321 until `2.0.0`, where hopefully we resolve the current ambiguity between the two interfaces.

> Either we ask users to always provide the network object, or we never do.  Optional arguments are easily forgotten, and therefore [IMHO], dangerous.
Two optional arguments is asking for trouble... Should we apply this change set directly to 2.0.0 instead?