status codes

Thanks for the great library; I'm enjoying tinkering with my WICED!

It would be neat if the AdafruitHTTPServer object was able to set the status code of the response within a dynamic callback handler in addition to (or instead of) generating the content. For instance in the example file_not_found_generator, one might hope to do something like the following:

```c++
...
void file_not_found_generator (const char* url, const char* query, httppage_request_t* http_request)
{
(void) url;
(void) query;
(void) http_request;
httpserver.sendStatus(HTTP_STATUS_NOT_FOUND);
}
...
```

Alternatively, one might desire access to some type of `httppage_response_t` that could be manipulated in more advanced ways.