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

- Adds -[NSArray(EKStuff) occurencesOfEachElementInArrayByUsingDictionary] which is more efficient than original occurencesOfEachElementInArray.
- BDD for #40
Merge pull request #42 from stanislaw/master

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