Fix deprecations

[jonnyijapan]

This lib needs deprecations fixed:

/JSONKit-master/JSONKit.m:680:12: 'isa' is deprecated
/JSONKit-master/JSONKit.m:680:5: Assignment to Objective-C's isa is deprecated in favor of object_setClass()
/JSONKit-master/JSONKit.m:931:17: 'isa' is deprecated
/JSONKit-master/JSONKit.m:931:5: Assignment to Objective-C's isa is deprecated in favor of object_setClass()
/JSONKit-master/JSONKit.m:2598:77: Bitmasking for introspection of Objective-C object pointers is strongly discouraged

[benhoyt]

This is a dupe of #133 -- Issue #133

[jcbertin]

See also #158.