Add method to enable in-app updates even if installed from market

The current implementation blocks this to prevent developers from accidentally leaving it on, but in turn also blocks developers from intentionally enabling it.

It's not the most attractive implementation (would prefer being able to chain the method, but `UsageEvent` doesn't contain an instance and I didn't want to change the API), but it's simple and makes itself available for developers that need it.

The new field is of course defaulted to `false`, again to ensure it doesn't change anything for existing users.

Please let me know what you guys think! Would really prefer to have first-party support for this rather than have to clone the SDK locally (as recommended [here](http://support.hockeyapp.net/discussions/questions/5646-is-it-possible-to-mix-using-hockeyapp-and-google-play-alphabeta-channels)).

 ```java
private void checkForUpdates() {
   UpdateManager.enableUpdateFromMarket(true);
   UpdateManager.register(this, APP_ID);
 }
```