Make sure we only add the last TxOut of created transactions as an unspent
output when that is a change output
Add rollback last output functionality

Note that this does not include the transaction finalization which is
part of the rollback last output procedure in the opentransactions
wiki.
Replace 32 zeros with a byte array
Comment what the purpose of the rollBackLastOutput function is
Fix comment
Rename lastOutput to removed for consistency
Rename createCredits to createFakeCredits for clarity
Make checkTxInputs also check TxIn
Clarify that the pkScripts are not valid.
Check value of TxOuts as well
Add popOutput test