+create: and a compound disposable on <RACSubscriber>

This replaces:

```objc
@protocol RACSubscriber
- (void)didSubscribeWithDisposable:(RACDisposable *)disposable;
@end

@interface RACSignal
+ (RACSignal *)createSignal:(RACDisposable * (^)(id<RACSubscriber> subscriber))didSubscribe;
@end
```

with:

```objc
@protocol RACSubscriber
@property (nonatomic, strong, readonly) RACCompoundDisposable *disposable;
@end

@interface RACSignal
+ (RACSignal *)create:(void (^)(id<RACSubscriber> subscriber))didSubscribe;
@end
```

Besides a more idiomatic name, and less need to explicitly specify block return types, this has some important consequences:

 1. The subscriber passed into the block now clearly represents _one_ subscription. It won't (directly) be a `RACSubject`, for instance. This means we can combine `RACSubscriber` and `RACPassthroughSubscriber`.
 1. Having a disposable _always_ in existence makes it easier to terminate synchronous signals early.
 1. You can watch `subscriber.disposable.disposed` to know whether to cancel, obviating any need for explicit disposables in many cases.
 1. Less memory load, because there's an existing compound disposable that can be used instead of creating new ones.
 1. The behavior of disposing a subscription is now more clear — everything in the compound disposable, and every _other_ subscription to that same `<RACSubscriber>`, will be disposed. This means you don't need to hold onto disposables from `-subscribe:` within `+create:`. (See the code for examples.)

**To do:**

 - [x] Clearly mention the above stuff in `RACSignal`'s documentation
 - [x] Try removing `RACPassthroughSubscriber`
 - [x] Update subscription APIs for terminable synchronous signals
 - [x] Remove `RACSubscriptionScheduler`
 - [x] Re-fix #94 with the new APIs
 - [x] Test `-subscribeSavingDisposable:…`
 - [x] Test not saving the disposable of `-subscribe:` within `+create:` 
 - [x] [CHANGELOG](https://github.com/ReactiveCocoa/ReactiveCocoa/blob/subscriber-disposal/CHANGELOG.md#simplified-signal-creation-and-disposal)