Major code rewrite / cleanup. closes jdg#56 jdg#60

- MBProgressHUD now requires LLVM 3+

- instance variables moved to the class implementation or removed (using synthesized instance vars)
- using KVO instead of custom accessors
- completely rewritten layout and UI buildup code
- more concise arc conditionals
- added MBProgressHUDModeText (+ example)
- various documentation fixes
- migrated to @autoreleasepool
- general source code cleanup and restructuring
- removed deprecated delegate callback
- version bump to 5.0