Automatically add UILaunchImages key to info.plist

* Automatically add UILaunchImages key to info.plist if portrait launch images are detected and user hasn't specified launch image configuration
* Added various portrait launch images, including for iPhone 6 and iPhone 6 Plus

With this commit, creating a RubyMotion project with the iOS template and running `rake device_name="iPhone 6"` or `rake device_name="iPhone 6 Plus"` will run the simulator at the native iPhone 6 and iPhone 6 Plus resolutions (points) rather than the zoomed 320x568 points.

Unfortunately, the launch image file names matching is rigid – but harmless since user can override UILaunchImages in the app Rakefile – and only works for portrait. Would be much better if detection can be done by examining the image dimensions directly, but that would introduce 3rd party dependency or involve fiddling with the image header data directly.

Note: the launch images are rgb(6, 6, 6). I can't get them to be rgb(0, 0, 0,) or rgb(3, 3, 3) and still make it through ImageOptim