Add InstanceType constants for new m3.medium and m3.large types.

The new instance types were announced here: http://aws.amazon.com/about-aws/whats-new/2014/01/21/announcing-new-amazon-ec2-m3-instance-sizes-and-lower-prices-for-amazon-s3-and-amazon-ebs/

Simplify the implementation of InstanceType.fromValue()