Adds Retina HD support to ImageLoader

JIRA ticket: https://jira.appcelerator.org/browse/TC-4752
Merge branch 'patch-6' of github.com:FokkeZB/titanium_mobile into timob-17734
[TIMOB-17734] Set up correct scales and filenames
Merge pull request #6146 from vishalduggal/timob-17734

[TIMOB-17734] iOS: ImageLoader doesn't support @3x