MasterPublicKey xpub = neutered extended public key

[louisinger]

This PR changes the masterPublicKey of MasterPublicKey (and Mnemonic)

• instead of returning baseNode.publicKey as masterPublicKey, return the neutered version of baseNode baseNode.neutered.
• same keys and addresses are generated, only the masterPublicKey returned by Mnemonic and MasterPublicKey identites are different.
• neutered master extended key is the master xpub of the bip32 node

small fix related to baseDerivation path: it closes #127

EDIT: the PR does not modify Multisig identities xpubs

@tiero @sekulicd please review