add MapPublicIpOnLaunch

fix #263 

This option is visible in the [subnet status](http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-ItemType-SubnetType.html), but only modifiable from another [API call](http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-ModifySubnetAttribute.html). 

```go
resource "aws_subnet" "main" {
    vpc_id = "${aws_vpc.main.id}"
    cidr_block = "10.0.1.0/24"
    map_public_ip_on_launch = true
}
```

Depends on: https://github.com/mitchellh/goamz/pull/95
Related to: #214, #252