Better timelocks handling, more consistent interface

Added

• TransactionFactory class that can instantiate either a Transaction or a SegWitTransaction from json, hex or bytes
• Address class now have a to_script method that generates the corresponding scriptPubKey
• More high-level interface to Locktime and Sequence classes
• Absolute and relative timelock solvers are now separated and take Locktime/Sequence as input so that the spend method can automatically fill in locktime and sequence numbers in the resulting signed transactions

Fixed

• Deserialization interface inconsistencies (see #42)

Backward compatibility

The following changes can break backward compatibility:

• TimelockScript has been renamed to AbsoluteTimelockScript
• Transaction now fails when trying to unhexlify a SegWitTransaction and vice versa
• BaseAddress has been renamed to Address, Address to ClassicAddress