Add categories description and Car example.
Add description and example of how to simulate protected methods.
Add extensions description with example.
Add much more to the protocols section.
Add much more to the protocols section.
Merge branch 'objective-c-protocols' of github.com:levibostian/learnxinyminutes-docs into objective-c-protocols
Merge pull request #486 from levibostian/objective-c-protocols

Objective-C protocols, categories, and extensions