Replaced Zlib with miniz

I replaced zlib with miniz ( https://code.google.com/p/miniz/ ). Since miniz is a single C file, end-users don't have to worry about setting up another external library. This obviously isn't a big deal on Linux, but on Windows/OSX/iOS/Android it can be annoying.

Miniz has a zlib-compatible API, so all I needed to do was include it and add it to the build.