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.
Merge pull request #159 from jimmysong/158

Issue #158: Add SPV when using electrum
async utxo fetcher
ComposedTxSpec, fee handling refactoring
added logging
async utxo fetcher update notification
colordef test fixes
p2ptrade refactoring
fix ComparableMixin
added basic tx history
receive history entries
init
add _get_chain
trade history entries, p2ptrade fixes
add code for reorg
add get_merkle and get_header
full compatibility with electrum
added history icon
make sure only one color per asset is possible
faster exit