Fix image loader on IE; add loader test

`obj.src = ""` triggers the error event on IE while it doesn't on webkit
so adding a check for the type of browser fixes the error.

See #549 for more details.
Merge branch 'feature/fix-IE-assets-loading-error' of git://github.com/potomak/Crafty into potomak-feature/fix-IE-assets-loading-error

Conflicts:
	Gruntfile.js