Added -rac_signalForSelector:

The simplest possible implementation of #267.

It doesn't address a lot of the use cases from the issue, but it _is_ pretty nice:

```objc
RACSignal *mouseExited = [[self rac_signalForSelector:@selector(mouseExited:)] mapReplace:@NO];
RACSignal *mouseHovered = [[[self
	rac_signalForSelector:@selector(mouseEntered:)]
	map:^(id _) {
		return [[[[[RACSignal
			interval:1]
			deliverOn:RACScheduler.mainThreadScheduler]
			mapReplace:@YES]
			take:1]
			takeUntil:mouseExited];
	}]
	switchToLatest];

RACSignal *showLulz = [RACSignal merge:@[ mouseExited, mouseHovered ]];
```

I don't think this precludes a more complete solution, but it's better than doing it manually.