Add simulation starter code

sim/.gitignore

sim/LICENSE.md

sim/README.md

sim/bit-array.js

sim/btc.js

sim/btc/blockchain.js

sim/btc/inventory.js

sim/btc/mempool.js

sim/btc/miner.js

sim/btc/transactions.js

sim/d3.v3.min.js

sim/goog/array/array.js

sim/goog/asserts/asserts.js

sim/goog/base.js

sim/goog/bootstrap/nodejs.js

sim/goog/bootstrap/webworkers.js

sim/goog/debug/error.js

sim/goog/deps.js

sim/goog/dom/nodetype.js

sim/goog/object/object.js