Identifiers: Prefix lowercase; remove the dash; use other version byte.

- Reason for lowercase prefix: "O" might be confused with "0" (is not
in the b58 alphabet for that reason).
- Reason for removing the dash: be able to double click and select the
whole identifier on all systems.
- Reason for the new version byte: eliminate the (small) risk that
someone uses the part after the prefix as a valid Bitcoin address.
Merge pull request #370 from benma/base58

Identifiers: remove the dash, use other version byte.