sx.py documentation improvements (squashed, 10 commits)

* example for sx-scripthash
* add example for rawscript
* split out sx blockchain commands that query obelisk, versus those that hit blockchain.info and blockexplorer
* note that sx balance is in satoshis
* put showtx in a new TRANSACTION PARSING category, to emphasize that this command can be run with no network/blockchain connection
* improve help message for fetch-transaction
* improve help message for show-tx, uses actual transaction in block chain rather than txfile.tx that user doesnt have
* move scripthash to new MULTISIG ADDRESSES category
* improve error message for sx rawscript
* move validtx into BLOCKCHAIN QUERIES category
Unify wording for BLOCKCHAIN UPDATES commands.

Help now reads:

BLOCKCHAIN UPDATES
   bci-pushtx                 Send tx to blockchain.info/pushtx.
   broadcast-tx               Send tx to bitcoin network.
   ob-broadcast-tx            Send tx to obelisk server.
   sendtx                     Send transaction to a single node.
sx.py documentation improvements (squashed, 4 commits)

* move showscript into TRANSACTION PARSING
* consolidate CREATE TRANSACTIONS category
* move showblkhead out of BLOCKCHAIN QUERIES, now BLOCKCHAIN QUERIES is truly all commands that touch the network
* finish consolidating CREATE TRANSACTIONS category