Add response time functionality

Added the ability to have a request pause before returning:

```objectivec
        stubRequest(@"POST", @"https://example.com/say-hello").
        withHeader(@"Content-Type", @"text/plain").
        withHeader(@"X-MY-AWESOME-HEADER", @"sisisi").
        withResponseTime(1).
        withBody(@"Adios!").
        andReturnRawResponse([@"HTTP/1.1 200 OK\nContent-Type: text/plain\n\nhola" dataUsingEncoding:NSUTF8StringEncoding]);
```
In this case the request will pause for one second before returning the response.