Changed `id` return type to `instancetype`

I changed the return type of the class constructors from `id` to `instancetype`. This enables you to track events without creating a `mixpanel` instance variable.

So, instead of using:

```
Mixpanel *mixpanel = [Mixpanel sharedInstance];
[mixpanel track:@"Clicked Button"];
```

You can save a line and use: 

```
[[Mixpanel sharedInstance] track:@"Clicked Button"];
```