Add GetTransactionByHash request, cleanup some features

[SirensOfTitan]

Full changelog:

• Added a param to set HTTP headers for requests. This allows, for example, basic HTTP Auth to be used.
• Upgraded all used pods.
• Added GetTransacitonByHash request.
• Updated Transaction to be up-to-spec: blockNumber can be null if it's still pending (i.e. still be transmitted).
• Ensured that we still flush websocket requests that are pending if the socket is connected.