Segwit poc

[julia-zack]

This branch was created to make the change the fed address into a segwit-compatible one.
For now,

1. Create a new method (called createP2SHP2WSHOutputScript) in ScriptBuilder class that creates the output script for a P2SH-P2WSH
2. Add a test to compare our calculation for a P2SH-P2WSH address against the one obtained with electrum
3. Rename existing function sha256hash160 to simply hash160 and replace it where needed. This makes sense since hash160 by definition first makes a sha256 encryption.