Support for Temporary Security Credentials

This PR adds support for AWS' temporary security credentials:
http://docs.aws.amazon.com/STS/latest/UsingSTS/CreatingFedTokens.html

If you are an IAM user that only has access to  temporary credentials, you can use ` config/ec2.yml ` as is with the addition of `:aws_session_token `:

```
:aws_access_key_id: "YOUR ACCESS KEY"
:aws_secret_access_key: "YOUR SECRET"
:aws_session_token: "YOUR SESSION TOKEN"
:aws_params:
  :region: 'eu-west-1'
:load_balanced: true
:project_tag: "YOUR APP NAME"
```

This also allows the session token to be set as the environment variable ` AWS_SESSION_TOKEN `.

Fog supports this as an optional attribute in your AWS credentials:
https://github.com/fog/fog/blob/master/lib/fog/aws/credential_fetcher.rb#L20