Add stub Ledger module / class
Stub Ledger.add_transaction
Ledger is a list of Transactions

This implementation is bad, so we need some other test to smoke out the
problem. Transactions need to be unique even if they are identical.
Minor whitespace cleanup
Add test to clarify equality/is behavior
Add test to smoke out contains problem in Ledger
Use composition instead of inheritance of list
Override __contains__ and fix add_transaction
Merge remote-tracking branch 'origin/ledger'