QCKDSL: no 'qck_' prefix unless shorthand disabled

Users may now use Quick in Objective-C using the following syntax
(notice the lack of the `qck_` prefix):

```objc
QuickSpecBegin(DolphinSpec)

describe(@"a dolphin", ^{
  it(@"is the smartest animal in the ocean", ^{
    // ...
  });
});

QuickSpecEnd
```

To namespace the `describe`, `it`, and other macros with the `qck_`
prefix, users must `#define QUICK_DISABLE_SHORT_SYNTAX 1` before
importing the Quick header.

Addresses issue #66.