New @ObjectiveCConforms annotation

[yegor.k wrote on j2objc-discuss](https://groups.google.com/forum/#!topic/j2objc-discuss/DaDjhW4ePtA):

With the existing @ObjectiveCName annotation it is easy for a Java class to implement UIKit protocols, but it's impossible to mark the class as conforming to a given protocol. My suggestion is to use an annotation to specify the list of protocols the translated ObjC class is confirming to, for example:

```
@ObjectiveCConforms("UITextFieldDelegate")
public class MyClass
{
@ObjectiveCName("textFieldDidBeginEditing:")
void textFieldDidBeginEditing(textField: Object)
{
// Do Something
}
}
```

Now, as in the previous case, this is mostly (see below) optional: one can skip protocol specification and cast in assignment, but as with the previous case, this would remove the need for a cast and add ObjC type checking, which is always good.

Also, with such a new annotation, the ObjC "conformsToProtocol:" function will actually work correctly for Java classes; granted, most libraries don't actually check the protocol itself, only checking "respondsToSelector:", but some do (for example, Google Maps SDK is checking for conformance to GMSMapViewDelegate and the delegate won't work if conformity is not declared).