Paving the way to indeterminate progress indicator in SPILDemo app UI.
Determinate progress support.
Reversed the indeterminate property and removed use of internal observing.

- I think that reversing to a isDeterminate property makes more sense because it makes the indeterminate state the default.
- Replace use of internal key/value observing by simple custom setters which avoids a bunch of problems.
Replacing helper functions with 10.8+ API.
Project file upgraded by Xcode 6.1.
Converting to modern ObjC.
Converting to ARC.