Issue #158: Add SPV when using electrum

Created new file verifier.py which does the actual merkle tree verfication
Created unit tests for verifier.py
Added three calls to electrum.py in both ElectrumInterface and EnhancedBlockchainState:

get_merkle
get_header
get_height

Created VerifiedTxDb in txdb.py.
Updated unit tests for test_services and test_verifier.