Fix issue #17, and proper handle product name with space.

Fix issue #17 (https://github.com/krzysztofzablocki/IconOverlaying/issues/17).
Fix to proper handle product name with space, thus be able to write the result app icon image to destination.

As I tested it, my app icons experience the same issue as in #17 of "unknown critical chunk". It's probably caused by XCode's pngcrush that processes on that file and imagemagick won't recognize it anymore.

So I normalize it back (not matter it's actually pngcrushed or not) via xcrun, and process on that version further instead.
Here the shot from my fork build for this commit.
![photo](https://f.cloud.github.com/assets/673757/2491736/936f8158-b1fa-11e3-9628-351f1c78262e.PNG)