add method to BookShelf class to get distribution of cases.

[skewballfox]

we need to add a method similar to get annotation cases that returns a histogram we can use to figure out how are data is distributed amongst cases.

The key should be the case_id and the value an int which is a count, alternatively you could use a counter from collections and thus avoid manually instantiating and updating the count

[skewballfox]

switched from gathering the cases with sets to gathering them with counters, we now have the distribution data