Improve API around basic authorization

[gmarz]

This PR makes some improvements to how basic authorization is handled on the client.

Previously, basic authorization credentials needed to be specified on the URI:

var node = new Uri("http://user:password@localhost:9200");
var settings = new ConnectionSettings(node);

The above approach is still valid, however the following is now possible (and preferrred):

var node = new Uri("http://localhost:9200");
var settings = new ConnectionSettings(node)
    .SetBasicAuthorization("user", "password");

Credentials can also be specified at the request level:

var response = client.RootNodeInfo(r => r
    .RequestConfiguration(rc => rc
        .BasicAuthorization("user", "password")
    )
);

When determining what credentials to use, the client takes the following precedence (highest to lowest):

• Request level
• Connection settings level
• URI