Fixed Podfile setup for building the project from source.

- Merged `ParseUIDemo` target from separate xcodeproj into `ParseUI.xcodeproj`
- Rewritten `Podfile` to use only `ParseUI.xcodeproj` with 2 targets and link dependencies only with `ParseUIDemo`
- Specified only `-ObjC` in custom linker flags for `ParseUI` target (this was the source of the problem)
- Removed `ParseUIDemo.xcodeproj`

These changes fix #3, #4

cc @grantland, @bklimt