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.
True, I assume you mean the following:

``` javascript
isPrivate = (version === network.bip32.private)
assert(isPrivate || version === network.bip32.public, '...)
```