Route53 TCP healthcheck bug

When creating a TCP health check the Route53 API requires that resource path is omitted. Currently, boto requires the resource path to be specified. This returns the following error from the API

```
<ErrorResponse xmlns="https://route53.amazonaws.com/doc/2013-04-01/">
    <Error><Type>Sender</Type><Code>InvalidInput</Code><Message>TCP health checks must not have a resource path specificed.</Message></Error><RequestId>4bf97687-c58c-11e3-9c4f-b18f7840909a</RequestId>
</ErrorResponse>
```

The fix is to default resource path to none and exclude the element from the request if not set.