Introduce shadow keychain notion
KeyChainGroup's currentAddress/freshAddress vend P2SH addresses for married keychains
Married HD wallets: KeyChainGroup currentKey/freshKey will throw an exception
Married HD wallets: add a method to marry the keys

A new method addFollowingAccounts is intended to "marry" a given list of extended public keys to the zero account key of the active keychain.
KeyChainGroup now holds NetworkParameters object

It is used for constructing addresses for married keychains and to render keys' addresses in toString implementation.
Married HD wallets: Fix naming inconsistency - s/shadow/following/