Add tests to the wallet.py module.

This increases test coverage a lot.
Removed failing test for now

As discussed on IRC, it might be too early to write this test since the
behavior is not entirely defined yet.
don't use address_is_old in code that can be run offline
don't use address_is_old in code that can be run offline (again)
fix: new outputs format and payment requests
Merge pull request #749 from chrisglass/clean-tests-wallet

Add tests to the wallet.py module.