adds instance tags to ec2 instances in aws

This adds instance tags to the aws_ec2_instance. I am not sure if this is the syntax that you guys are wanting but I am open to changing it. As requested in #28

```
resource "aws_instance" "web" {
    ami = "ami-1234"
    instance_type = "m1.small"
    tags = ["Name:web", "Region:us-east-one"]
}
```