Adds Retina HD support to ImageLoader

JIRA ticket: https://jira.appcelerator.org/browse/TC-4752
[TIMOB-17734] Set up correct scales and filenames
Merge pull request #6147 from vishalduggal/timob-17734-34X

[TIMOB-17734] (3_4_X) iOS: ImageLoader doesn't support @3x