[faucet] Add a faucet application

Summary: This is a very simple faucet that will be used to send testnet coins. It is so simple that it can be used as an example on how to use our libraries. Also I don't think unit tests would make sense for this kind of application.

Test Plan:
Edit the configuration file then:

npm install
npm start

Try it out via curl requests like so:

curl http://127.0.0.1:18300/balance
curl http://127.0.0.1:18300/claim/<your address>

The logs are verbose so you can easily check what is happening.

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D16865