Make cryptoshi audit use standard Bitcoin signatures

`cryptoshi audit` was using some non-standard signature format. This patch replaces that with fully compliant Bitcoin signatures. I think that's better for future audits, so that other compatible tools can be written easily, such as [easy-audit](https://github.com/justmoon/easy-audit).

Changes:

* Include varint sizes in message serialization before hash
* Support for uncompressed pubkey addresses
* Remove address from signed message (not needed)