Declare category additions as readonly properties, instead of instance methods, for better Swift compatibility

[JRG-Developer]

This PR changes category additions to readonly properties, instead of instance methods.

This way, they are correctly converted to Swift computed properties automatically.

Otherwise, as with the current version, you'd need to declare obfuscated strings like this:

NSMutableString().p().a().s().s().w().o().r().d

With this PR accepted, you'll be able to declare obfuscated strings as intended like this:

NSMutableString().p.a.s.s.w.o.r.d

[coneybeare]

We used to have it like this, but I can't remember why we changed it, maybe subclassing reasons? Seeing as swift is the future, I am inclined to accept the PR, but it's been awhile since I've done any obj-c dev and want to hear from others about the potential pitfalls from removing instancetype

[RomanPodymov]

Hello @JRG-Developer
There are no UAObfuscatedString.h and UAObfuscatedString.m in the project. Please close this pull request.