Issue #78: apply_tx for txhash in mempool

added a way to make a ComposedTxSpec from pycoin_tx
added mempool transactions when you scan
Note mempool transactions have no guaranteed order, which doesn't affect adds into the utxodb but it does affect the deletes. We simply do all the deletes again at the end.
Merge pull request #102 from jimmysong/78

Issue #78: apply_tx for txhash in mempool