Alternate emscripten build flag for WebWorker compatibility

Flag -w for emscripten ./build.sh script will not use wrapper-start and wrapper-end files and will not include the window unload listener.
Fixed: Emscripten unload listener moved to separate file

Reordered build script so that the proxy code was added to both and then only the unload listener (in a separate file) was added to the non-webworker version.
Merge pull request #54 from DDMAL/webworker-compatibility

Alternate emscripten build flag for WebWorker compatibility