Fixed Windows building/generation errors

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.
Fix raking with an included Gemfile.apk

A problem would occur when your building directory had a space in it,
fixed by adding quotes around the paths inside the command line action.
Also fixed an issue where the Gem.paths would be set wrong, fixed by
specifying the GEM_PATH key of the gem_paths variable.
Check against android_home being nil before gsub
Fix can't modify frozen string error
Merge pull request #444 from CaptainJet/master

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.