amazon ec2 driver

This is a driver for AWS.  It uses the Ubuntu 14.04 amd64 AMI.  There are some assumptions made that could probably be refactored to allow for customization or easier use:

* VPC is required (subnet ID must be specified)
* A unique security group is created per machine and authorized for the requesting host only
* A unique key pair is created per machine
* A "pre-baked" Ubuntu 14.04 AMI with Docker 1.3.2 is default (can be changed)
* Default region is us-east-1 (can be changed)
* Default instance type is t2.micro (can be changed)
* Default root disk size is 16 GB (can be changed; must be >= 16)

Example Usage:

```bash
machine create -d aws
    --aws-access-key [key]
    --aws-secret-key [secret]
    --aws-subnet-id [subnet-id]
    docker-test-machine
```

Test Builds:

OS X: https://ehazlett.s3.amazonaws.com/public/machine/darwin/machine-dev-a252ee1
Linux: https://ehazlett.s3.amazonaws.com/public/machine/linux/machine-dev-a252ee1