Enable the IsStandard check.

Previously, IsStandard always returned true.  Now, only return true if
the script can, indeed, be parsed as standard script.  Name prefixes are
stripped before attempting that.
Keep unspendable scripts out of the UTXO set.

Scripts that start with OP_RETURN are unspendable.  Detect those and
don't include them into the UTXO set.
Implement SendMoney for multiple recipients.

Rewrite the wallet sending functions a bit to simplify sending to
multiple recipients.
Handle OP_RETURN tagging in the RPC interface.

Allow adding an OP_RETURN output via a fifth, optional argument to
"sendtoaddress".  Display such OP_RETURN tags in decoded transactions.
Update IsStandard to accept tagging transactions.

In particular, allow tagging after the hardfork (not necessary, but
ensures that no clients are around that still include the outputs into
the UTXO set), for up to 100 characters of length and with 0.01 HUC
locked.
Merge pull request #89 from domob1812/op-return

Enable storing "tags" to transactions with OP_RETURN scripts.