Married HD wallets: introduce shadow keychain notion

Pull request: #99
Based on design notes:
https://groups.google.com/d/msg/bitcoinj/Uxl-z40OLuQ/e2m4mEWR6gMJ
Improvement of getFollowingKeys

First of all, freshAddress was actually asking for a fresh key twice for non-married keychain. That was fixed by moving first call (needed only for married chain) inside the getFollowingKeys. As the latter now started to return all keys in a marriage and not only following ones, it was renamed to freshMarriedKeys.
Having all the keys in one block allows to do simple derivation path check to make sure keychains are in sync (as per @devrandom suggestion)
Prevent marriage of used keychain

 addFollowingAccounts method now has the check that active keychain has no keys in use. This would prevent divergence of derivation paths for followed and following keys. In future this behaviour should be replaced with some sort of key rotation.