Improvements to Transaction

[eordano]

This commit contains a series of modifications that where detected from @matiu's feedback:

• The addInput method was not being used when from was called with a non-p2sh output
• The change output had no actual reason not to be updated on each modification, as there is no private key cache that could be signing more often than what we wanted for performance issues
• Add a fee field to the toObject serialization

[maraoz]

LGTM

[maraoz]

please check test coverage: coverage/coveralls — Coverage decreased (-0.02%) to 96.89%1

[coveralls]

Coverage increased (+0.08%) to 96.99% when pulling e568a87 on eordano:transaction/changeupdate into c07ee19 on bitpay:master.

[coveralls]

Coverage increased (+0.08%) to 96.99% when pulling e568a87 on eordano:transaction/changeupdate into c07ee19 on bitpay:master.