CB-2606, CB-3571 Add support for <icon>, <splash>

This commit fixes the following issues:
CB-2606 Add support for <icon> elements in config.xml
CB-3571 Add support for <splash> elements in config.xml

Copys <icon> and <splash> assets from the config.xml file to
the approriate platform paths.

Uses ImageMagick if it's available to resize all images
from a source image.