Issue #87: Make a method that takes a pycoin tx and converts to a composedTxSpec

Unfortunately, there's no way to do this in txspec.py without the coloredcoincontext. This is because Address depends on whether we're on testnet or not.
Similarly, the call to blockchain_state's get_tx is unavoidable. That is how we get the actual value of the utxo.
Even in pycoin's own scripts/send.py, it uses a call out to blockchain.info in order to get the actual amount that's available in the utxo.

I'm not exactly sure how to test it or in what context a call like this would happen. Please let me konw what I can do to do that.
Merge pull request #88 from jimmysong/87

Issue #87: Make a method that takes a pycoin tx and converts to a compos...
mempool coloring update
Merge branch 'master' of github.com:bitcoinx/ngcccbase
ComposedTxSpec refactoring
fix
Issue #89: backward scan coloredcoinlib

Add a recursive function to scan all matching inputs before computing the colorvalues.
Each input in turn needs to have the colorvalues of the matching inputs as well.
Merge pull request #90 from jimmysong/89

Issue #89: backward scan coloredcoinlib
fix minor bug in output:  sometimes there is nothing to spend