Insert sigs in scriptSig in order of the signers

This way the first sig will be the one from LocalTransactionSigner, the
second sig will be the one from the next signer added to the wallet and so
on.
To achieve this, following keys should be added to the wallet in the same
order as signers. Pubkeys aren't sorted anymore when creating redeem
script, so now the order of the keys matter. The first pubkey in redeem script
will always be the pubkey from the local (followed) keychain, the rest are
in the same order as following keys passed to the Wallet.addFollowingAccountKeys.

I've tried to describe all the above in javadocs, although explaining
things in English is not my best skill. Let me know if you feel it's not enough
or simply wrong.