Support for 3x assets when running on the 6 Plus

Added support for extracting 3x assets, similar to how 2x assets are handled.
This also requires adding new launch screens so that iPhone 6 and 6 Plus run
in their native resolutions instead of the scaled iPhone 5 resolution. Without
that, iPhone 6 Plus reports itself as having 2x screen scale instead of 3x.

Tested by running the code on the iPhone 6 Plus simulator. Extracted assets
were named with the "@3x.png" suffix.

Fixes #27