Better count occurrences of each element in array (#40, #41).

Howdy!

I've added SpecHelpers.h with `benchmark` method. For example, you may use it to compare how both methods perform:

```objective-c
// create originalArray (like it is done in specs)
benchmark(100, ^{
    NSDictionary *result = [originalArray occurencesOfEachElementInArrayByUsingDictionary];

    NSLog(@"Result is %@", result);
});

benchmark(100, ^{
    NSDictionary *result = [originalArray occurencesOfEachElementInArray];

    NSLog(@"Result is %@", result);
});
```