Adding an iOS Framework target and fixing header visibilities for all targets.

This change allows code written in Swift to leverage the ZXingObjC library. The framework target has the same or more appropriate build settings as the iOS static library target.

The header visibilities for all targets were necessarily updated in order allow a consumer written in Swift to correctly link against the ZXingObjC iOS framework. Previously the set of header visibilities for the target could not be built against due to references in the top-level framework header (ZXingObjC.h) to headers that were Project and not Public visibility.