Define dust transaction outputs, and make them non-standard
CreateTransaction: return strFailReason on failure
Un-hardcode TX_FEE constants

Allow setting of MIN_TX_FEE / MIN_RELAY_TX_FEE with
-mintxfee / -mintxrelayfee

Default values are the same (0.0001 BTC).
Merge pull request #2577 from gavinandresen/fee_bandaid

Treat dust outputs as non-standard, un-hardcode TX_FEE constants