Minor Fixes

Just a couple of minor fixes to better align with Objective-C style and best practices. Highlights:

* Instance variables should never be used anywhere except the initializer. Getting the value of a property anywhere else should be done with dot syntax.
* `count` is a method, not a property - it should use bracket syntax and not dot syntax
* In Objective-C, you should never have a variable name like `c` - Objective-C should be closer to written English than other programming languages, and Xcode's code autocompletion helps make sure this isn't a hassle.
* Changed the Course class's `description` property to `courseDescription` so that it wouldn't clash with NSObject's `description` method, resulting in unexpected behavior.