[WIP - DO NOT MERGE] Add searchrawtransaction

This pull request adds `btcjson` support for the "searchrawtransaction" RPC command. This PR is related to conformal/btcdb#11, and is another PR towards the completion of conformal/btcd#190. 

The defaults for the new RPC command have been copied from the initial implementation in a fork of `bitcoind` (verbose=true, skip=0, count=100).