Lift with a signal of tuples of arguments.

I often have a signal sending tuples whose values are to be the arguments in some message; typically, I send these using `-rac_liftSelector:withSignals:` (vs. e.g. `-subscribeNext:`).

Unpacking these signals-of-tuples into the arguments to `-rac_liftSelector:withSignals:` distracts from the intent of the code, and is needless—RAC just goes and re-packs them into tuples anyway. Therefore, I’ve added the `rac_liftSelector:withSignalOfArguments:` method (which is called into from `-rac_liftSelector:withSignalsFromArray:`) to clarify this use case.

**Usage:**

```objective-c
RACSignal *friendsAndNames = [RACObserve(self, friends)
	map:(Friend *friend) {
		return RACTuplePack(friend, friend.name);
	}];

[friendsByName rac_liftSelector:@selector(setObject:forKey:) withSignalOfArguments:friendsAndNames];
```