Async

* Makefile bugfix
* Async calls (either deferred or truly async, configurable)

**Todo**

- [ ] Test on Android and iOS
- [ ] Test on cocos2dx

@tokutake @MiLk @aogilvie would love if one of you (or all!) would be able to take some time and test this on the environments you care about. I will also be doing some tests on my side. Let me know if you find anything broken in the meantime
What about the following?

```c++
        std::launch policy = doAsync ? std::launch::async : std::launch::deferred;
        return std::async(policy, [this, name, params]{
            return Call(name, params);
        });
```