Add optional timeout to JSON-RPC daemon/wallet

[bitmario]

This PR adds a timeout argument to JSONRPCWallet and JSONRPCDaemon which is applied to all requests, fixing #18. The timeout defaults to None so this change is backwards compatible.

The timeout is passed directly to requests.post so it can be simply a number of seconds or a (connect timeout, read timeout) tuple (see https://3.python-requests.org/user/advanced/#timeouts).

No special exceptions are raised. If needed, the caller will have to handle the Timeout exception from requests, similarly to what currently happens for other types of connection errors.

[dethos]

@bitmario can you please add this extra feature to the Readme's list of changes?
I would also add the warning there, highlighting that the operation might succeed even if the timeout is triggered.

[bitmario]

Thanks @dethos, I have added a note to the readme.

[lalvarezguillen]

👍