more currency work

- CCurrency class to keep track of all currency info, stored in the
CTxDB
- read/write currency from CTxDB
- refactor checking valid string, use it for currency name as well
- enforce spending all newly minted coins
- check valid currency name
- check don’t mint existing currency
- check sig when remint/chowning
- fix SignatureHash, do sensible things for other sighash types