Fixed memory leak with NSURLSession

It looks like holding the reference for the NSURLSession until you no longer need it may solve [issue 1](https://github.com/iOSDevTraining/books-and-movies/issues/1) leak if in dealloc we make sure to cancel and invalidate using [<code>[self.urlSession invalidateAndCancel]</code>](https://developer.apple.com/library/ios/documentation/Foundation/Reference/NSURLSession_class/Introduction/Introduction.html#//apple_ref/occ/instm/NSURLSession/invalidateAndCancel)

A better solution would be to have only one session object per application lifetime and manager it on another class responsible for handling requests.

I also fixed some stuff I noticed regarding a known bug in iOS7 with the Separator of <code>UITableViewCells</code> and added a  <code>gitignore</code> file.