Backdoor

## Summary
- Backdoors support (identical to iOS)
- Ignore build files
- New static method for `Result` class

## Backdoor (#291)
This is identical to bakdoor in iOS.
Backdoor methods must all have the same signature
```
public String backdoorMethod(String arg);
```
They must be methods of the main `Application` subclass (aka main activity)
```
import android.app.Application;

public class MyApplication extends Application {
    /* ... */
    public String backdoorMethod(String arg) {
        return "";
    }
}
```

To implement backdoors added new 'backdoor' action and returning backdoor result in "bonusInformation", also implemented `backoor` ruby API to behave just like iOS version.

New file: `actions/application/Backdoor.java`.

## Other
- Add new method to create instance of successful result with message (`Result.java`)
- Ignore `ruby-gem/test-server/out`
- As a byproduct - remove trailing spaces and fix blank lines in touched files.