Update SDK to use Session when initializing service clients

[jasdel]

This change updates the way the SDK service clients are created.
Instead of taking an optional aws.Config pointer value the SDK now
requires a Session to be provided to the service clients when creating
instances. This session takes over for the global
defaults.DefaultConfig. Instead of being a global value the defaults
values are collected and set to a Session when session.New() is called.

To migrate to this change you'll need to add a session value to each
place a service client is created. Naively this can be just
s3.New(session.New()) in place of s3.New(nil). If aws.Config value
was passed in your code would become, s3.New(session.New(), cfg). If
you used default.DefaultConfig you can create a new session, set the
config you used on the default config like session.New(myCfg) or
sess.New(); sess.Config.LogLevel = aws.LogLevel(aws.Debug).

Examples:

    // Create a session with the default config and request handlers.
    sess := session.New()

    // Create a session with a custom region
    sess := session.New(&aws.Config{Region: aws.String("us-east-1")})

    // Create a session, and add additional handlers for all service
    // clients created with the session to inherit.
    sess := session.New()
    sess.Handlers.Build.pushBack(func(r *request.Handlers) {
        // Log every request made and its payload
        logger.Println("Request: %s/%s, Payload: %s", r.ClientInfo.ServiceName, r.Operation, r.Params)
    })

    // Create a S3 client instance from a session
    sess := session.New()
    svc := s3.New(sess)

    // Create a S3 client with additional configuration
    svc := s3.New(sess, aws.NewConfig().WithRegion("us-west-2"))

In addition to this change there were a few other changes in this update.

• aws/service:Service was renamed to aws/client:Client.
• aws/service/serviceinfo:ServiceInfo was renamed to aws/client/metadata:ClientInfo
• aws:DefaultRetries was renamed to aws:UseServiceDefaultRetries
• aws/ec2metadata now takes a session and aws.Config like all other service clients
• aws/credentails/ec2rolecreds:EC2RoleProvider now requires an ec2metadata.EC2Metadata and will not default to creating one.
• service/s3/s3manager:Downloader now requires a Session or S3 client instance, and also can take functional options setting the Downloader's options.
• service/s3/s3manager:Uploader now requires a Session or S3 client instance, and also can take functional options setting the Uploader's options.

[jasdel]

Released this change in tag v0.10.0