Replace the NSNotificationCenter mechanism in the quote creation flow

[fellipecaetano]

After reading through the code, it caught my eye that the quote creation flow uses NSNotificationCenter to notify the list of quotes that a new quote was created. I not a very big fan of NSNotificationCenter, and some of my reasons are described here.

I made a small fix replacing the NSNotificationCenter mechanism by a new one using a segue identifier and a quote creation handler (implemented as a closure in the CreateQuoteTableViewController). I think we can discuss the validity (and code quality) of the fix and if it fits the purpose of the example.

Thanks!