Adds option not to clear the build folder.

Setting the variable `clearBeforeBuild` to false doesn't regenerate the
build folder.
Added high-res icons for android and iOS
Merge pull request #102 from serratus/hires-icon-update

Adds high-res icons for android and iOS.
Merge pull request #100 from logankoester/gp-no-clean

Adds option not to clear the build folder.
Updates ncp and grunt-contrib-clean dependencies