Add small-data OP_DROP transactions as standard transactions.

This change is longer than preferred, because mTemplates is a map, and as
such, the notion of "variant of TX_PUBKEYHASH" is poorly supported.

TX_PUBKEYHASH_*DATA was added, but is largely to quiet compiler warnings,
because it is mapped to TX_PUBKEYHASH everywhere, including Solver()'s
transaction-type return value.

Intended to enable use cases such as Mike Hearn's distributed bond markets: https://bitcointalk.org/index.php?topic=92421.0