Add authentication

[watzon]

Several RPC servers require authentication to access them, such as is the case with litecoind and similar. It would be nice if you could do something like client.basicAuth("username", "password"), and have it generate the necessary authentication header.

[rjframe]

The server needs to be able to support requiring clients to authenticate too; currently an HTTP transport still needs to be written before either can be done (basic auth would be an HTTP transport-level authentication).

Thanks.

[watzon]

Yeah I figured as much, but I figured I'd get the issue listed here. Great project.