Feature/txproposal

This implements
  - PrivateKey for holding and deriving BIP32 priv keys
  - and TxProposal for holding and sharing TxProposal between the copayers.

Still some refactor need to be done in TXP, and same methods are missing, but it can merge p2sh signatures between pairs already. 

Next step: integrate then with @cmgustavo's wallet and with the Ux.