Add dumppubkey RPC command

This adds a dumppubkey RPC command. It closely resembles the dumpprivkey command, but it outputs the public key of an address encoded in hexadecimal rather than the private key in base58. (bitaddress.org, sx, and bitcoin's createmultisig and addmultisigaddress RPC commands are all compatible with this public key encoding.)

This is intended to help facilitate multisig transactions. Multisig addresses require a public key from each participant. There's no simple way using the existing Bitcoin client for participants to figure out the public keys of their addresses without this feature.