Add more description to @property getters and setters.
Add getters and setters examples.
Add data types with examples.
Add examples of methods to work with NSSets.

Add note of NSSets being unordered.

Remove for loop to iterate on NSSet. Moved for loop to loop section.
Add more data type examples.

Add NSMutableSet examples.
Add instance variable definition examples.
Merge pull request #470 from levibostian/objective-c-properties

[objective-c/en] Objective-C property examples