Address multiple issues triggering bugs in iOS Native SDK and Titanium SDK

This PR addresses the following open tickets with the APSHTTPClient library:

iOS: APSHTTPClient should use ARC
https://jira.appcelerator.org/browse/TIMOB-17098

OS: APSHTTPClient should not advertise protocol adherence in public headers
https://jira.appcelerator.org/browse/TIMOB-17100

OS: APSHTTPClient should not advertise ivars in public headers
https://jira.appcelerator.org/browse/TIMOB-17101

iOS: APSHTTPClient should use properties with attributes instead of ivars
https://jira.appcelerator.org/browse/TIMOB-17105

iOS: APSHTTPClient should move private state into properties on anonymous categories
https://jira.appcelerator.org/browse/TIMOB-17106

iOS: APSHTTPClient should use "Modern Objective-C"
https://jira.appcelerator.org/browse/TIMOB-17116

iOS: APSHTTPClient should use type safe enums
https://jira.appcelerator.org/browse/TIMOB-17117

iOS: APSHTTPClient should delete APSHTTPOperation
https://jira.appcelerator.org/browse/TIMOB-17118

iOS: APSHTTPResponse should should not advertise ivars in public headers
https://jira.appcelerator.org/browse/TIMOB-17125

iOS: APSHTTPResponse should use type safe enums
https://jira.appcelerator.org/browse/TIMOB-17126

iOS: APSHTTPRequest should should not advertise ivars in public headers
https://jira.appcelerator.org/browse/TIMOB-17129

iOS: APSHTTPRequest should use type safe enums
https://jira.appcelerator.org/browse/TIMOB-17130

iOS: APSHTTPOperation crashes app randomly
https://jira.appcelerator.org/browse/TIMOB-17131