Allow global configuration to include service-specific settings.

Example:

    AWS.config.update({s3: {endpoint: 'localhost'}});
    var s3 = new AWS.S3(); // will hit the localhost endpoint
    var other_s3 = new AWS.S3(); // this one too
Merge pull request #387 from aws/service-specific-global-config

Allow global configuration to include service-specific settings.