Fixed libpng compile issue on Windows

Fixes KhronosGroup/glTF#32

We now build with libpng regardless of platform. If we find it we define USE_LIBPNG and it is used correctly. If we can't find it cmake issues a warning and we behave like we did before on Windows.

Below are some instructions we can use to add something to the wiki

In order for this to work on Windows the user must define CMAKE_INCLUDE_PATH & CMAKE_LIBRARY_PATH environment variables. I added them in my .bash_profile for git bash like this

export CMAKE_INCLUDE_PATH="/c/Program Files/zlib:/c/Program Files/libpng"
export CMAKE_LIBRARY_PATH="/c/Program Files/zlib:/c/Program Files/libpng"

The library and include directories can obviously be different, but I just dumped everything in the same directory.

The user also has the option of adding all directories cmake needs to the system's PATH environment variable.