Cedar.h imports SpecHelper to make it behave more as expected

As per the CocoaPods installation instructions on the wiki, spec files need to add the line `#import <Cedar-iOS/SpecHelper.h>` to use Cedar. However there is a general expectation that to use a library, you import a header with the same name. This change allows for `#import <Cedar-iOS/Cedar-iOS.h>` which will reduce confusion.