add set_private_key method to update a client to enable authenticated endpoints.

[8ball030]

          i just picked that line to show that there's currently only one way to set the private key, at init time

would be nice if it could be added after init, so we can add it without having to re-instantiate? i guess a minor benefit

instead of client = client(private_key,...) to access private functions you could do client.set_private_key(private_key)

Originally posted by @wakamex in #16 (comment)