Fixes #295

Updated OBASelectorJsonDigesterRuleTarget enum to use modern syntax
Updated the OBASelectorJsonDigesterRule.h to return instancetype & added a note explaining target memory paradigm

Simplified the implementation of OBASelectorJsonDigesterRule & replaced ivars with properties.    ivar by default maintains a strong reference, this was causing a retain cycle.
Merge pull request #296 from beliyd/dev/fix/issue-295

Fixes #295