Feature/create tx

This implements the method #create in Transactions. Its aims to provide a much simpler interface to create new transactions. Some auxiliary functions were added in Transaction and Address. The test were updated to cover the new code.

The raw output transaction from some given inputs are compared in the tests with the (hardcoded) output of createrawtransaction from bitcoind. The parameters to generate those outputs in bitcoind are included for validation.

~~In a future PR the transaction signing (#sign) will be implemented~~
`.sign` is provided to sign a transaction using a given key array.