Create boolean-circuit representations and factories

[sappenin]

This is a tracking ticket for the concept of "boolean circuits" in the crypto-conditions library.

The idea would be to create an interface called BooleanCircuit that would have various imlementations. For example:

• OrGate: Contains a 1-of-2 Threshold condition, allowing it to operate like an OR gate.
• AndGate: Contains a 2-of-2 Threshold condition, allowing it to operate like an AND gate.

Use something like BooleanCircuitFactory, this library could be used to construct boolean logic circuit based upon crypto-condition primitives.

References

• Inspiring Issue Discussion: https://github.com/bigchaindb/cryptoconditions/issues/94
• Gist containing some sample code (but containing incorrect interfaces): https://gist.github.com/sappenin/fade24278f1a90df22a9e05409a14e91

[sappenin]

This issue was transitioned from the Quilt library's https://github.com/hyperledger/quilt/issues/108.