Fix building on Windows caused by mixed file separators. Also pre-compressed the included PNGs with optipng, despite included pngcrush during rake for increased loseless file compression.

Mixed file separators using both "\\" and "/" caused the finding of
android platforms and android build tools to return nothing found on
Windows, so gsub-ing the "\\" with "/" has fixed the issue, though I
don't know if this breaking anything on Linux/Mac. Also compressed all
the included images.