Married HD wallets: introduce shadow keychain notion

As per [married wallets design notes](https://groups.google.com/forum/#!msg/bitcoinj/Uxl-z40OLuQ/e2m4mEWR6gMJ) a shadow key chain (the one that follows another keychain) was introduced to `KeyChainGroup`.
From client's perspective to make married wallet one has to call wallet's `addFollowingAccounts` supplying xpub form of extended public keys of other signing parties. After that `currentAddress`/`freshAddress` calls will start yield P2SH addresses made of corresponding regular and shadow key chains keys.

Shadow key chain is a `DeterministicKeyChain` and is being serialized along with the normal `DeterministicKeyChain`s with the only difference that it has a reference to "followed" keychain and has no seed.

Add a brief comment here explaining what the map keys and values are here: the map keys I assume are the root/account keys of the following/third party chains?