DPCalendarMonthlyView collectionView events and iconEvents now are on demand

Hi, first of all very nice repository and very useful for the project that I am currently working on.

I had to adjust a little bit the way that events and icons events are pulled by the collection views.

My project demands a calendar with more than 2000+ events and processing all of them in a background queue every time that a new event needs to be included is not feasible.

I've changed the class in order to only return the events array that the collation view is asking for a given date.
It become a way faster,  less code and doesn't need to process anything in BG.
 
Also I've modified few places to the Objective C best practices. Hope you like I've done.

Cheers, Flavio