Create UserDefaults constants to avoid the widespread use of strings

- Add UIUserDefaultsKeysConstants.h and import it in the .pch
- Refactor all code uses of NSStrings as keys of NSUserDefaults. This makes code reuse and future refactor safer
Merge pull request #11 from Cocoanetics/fcy/user-defaults-keys-as-constants

Create UserDefaults constants to avoid the widespread use of strings