Add constructor to ExtKey to create an ExtKey object from a private key and chain code.
Add constructor to ExtKey to create an ExtKey object from a private key and chain code.

HermanSchoenfeld/NBitcoin,NicolasDorier/NBitcoin,dangershony/NStratis,stratisproject/NStratis,thepunctuatedhorizon/BrickCoinAlpha.0.0.1,lontivero/NBitcoin,MetacoSA/NBitcoin,MetacoSA/NBitcoin