adding get call

one issue is that I'm parsing duration into a string because json.Unmarshal has issues marshalling "" into an int. if I add a string annotation I get this error: "json: invalid use of ,string struct tag". if I don't add the annotation I get a twilio error. we can talk more in person.
like this:
```go
var m Response
if true != validateCallSid(sid) {
    return m, errors.New("Invalid sid")
}
err := act.getCall(fmt.Sprintf(getUrl, act.AccountSid, string(sid)), &m)
return m, err
```