Updated config settings to be read/write

I added setters to the properties in ClientConfiguration to allow them to be modified at run-time.  I've got a winforms app that makes an API call to get the key and secret, and I needed to be able to instantiate and configure a client at run-time.

For example:

```c#
public class Google
{
    private GoogleClient client;

    public Google()
    {
        var secrets = SecretsClient.Get("google");

        var config = new ClientConfiguration()
        {
            ClientId = secrets.Key,
            ClientSecret = secrets.Secret,
            Scope = @"https://www.googleapis.com/auth/userinfo.profile https://www.googleapis.com/auth/userinfo.email",
            RedirectUri = "urn:ietf:wg:oauth:2.0:oob"
        };

        client = new GoogleClient(new RequestFactory(), config);
    }
}
```