Implement new send and sendAsync api

This commit adds a new send api which is now located on the QTdRequest for convenience So it now should be enough to just do something like

QScopedPointer<QTdRequest> req(new QTdRequest);
req->send();

Which will internally pass it on to QTdClient::send()

There is also a new sendAsync api on the QTdRequest that will return a QFuture<QTdResponse> so that you can wait on a response for a request and then unmarshal the data into the desired structure

QFuture<QTdResponse> resp = req->sendAsync()
await(resp) // or you can use QFutureWatcher
if (resp.result().isError() {
    qWarning() << resp.result().errorMessage();
}
myStructure->unmarshalJson(resp.result().json());

As you can see above there is also an await() helper located in utils/await.h which will allow you to wait inline without blocking the mainthread. The await helper takes a second argument for a timer to abort the wait after a certain amount of time.

The current api of passing the request to QTdClient::send is still supported so this isn't a breaking change.