Feature/bitcon core support

[RiccardoBiffi]

Submission for the assignment 6 - Plan B Network Tech School.

General changes

• added unsubscribe methods on Electrum provider
• added Electrum provider tests
• added Bitcoin Core provider
• added Bitcoin Core provider tests
• added a package dependency (ZeroMQ)
• some updates on test-helpers

Choices

• A new files has been added to .gitignore: package-lock.json, as of what we said on Telegram
• The module now exports two distinct providers: ElectrumProvider and BitcoinCoreProvider.
• Bitcoin Core provider subscriptions are managed with ZeroMQ
• Only one ZeroMQ socket is used. All messages go through it with a different topic to distinct them.
• The socket port is 28334. The standard one should be 28332, but the suggested bitcoin.conf file uses three different ports starting from 28334. I chose the first one.
• The ZeroMQ socket emits different events depending on the topic. The Bitcoin Core provider listens to the events and their handler contains the logic. This way I can assure only one socket is opened and it is listened only by one process.
• I changed the name of _getBalance() to getBalance() in both Electrum and Bitcoin Core providers, with the intention to signal it as a public method.
• New test-config schema: the schema has been updated to reflect the presence of the new Bitcoin Core provider. It is now a dictionary with an entry for each provider and the needed properties.

[CLAassistant]

All committers have signed the CLA.