[wip] aws driver

This is an initial driver for AWS.  It is not ready to merge as not everything is implemented.

This is to get feedback.  It uses the Ubuntu 14.04 amd64 AMI.  There are some assumptions made that could probably be refactored to allow for customization.  These are the assuptions:

* VPC is required (subnet ID must be specified)
* AWS key pair must exist
* AWS keyfile is required and must exist already (this could be refactored to be generated and used during creation)
* Security Group: a security group ID must currently be specified and only a single group is allowed (this could be refactored to allow group names and multiple)
* Security Group must also be pre-configured to allow access (authorizations will be added soon)

Example Usage:

```bash
machine create -d aws --aws-access-key [key] --aws-secret-key [secret] --aws-subnet-id [subnet-id] --aws-security-group-id [sg-id] --aws-key-name [keypair] --aws-key-path [/path/to/keyfile.pem] docker-test-machine
```

Signed-off-by: Evan Hazlett <ejhazlett@gmail.com>