Issue #26: Subclass BlockchainState and grab raw transactions from electrum
           in case local bitcoind is not txindexed

This creates an EnhancedBlockchainState in electrum.py.

This necessarily uses blockchain.info to find the height of the txhash
since otherwise, we won't be able to look up any raw transactions in electrum.
Merge pull request #41 from jimmysong/26

Issue #26: use electrum for get_raw_transaction