[WiP] Adding Mac OS X Support

In order for smashbattle to also work on macs, I started to investigate how this was best done. Turns out that only SDL is missing from a standard mac and installing it is easy (on a dev machine). All I then had to do was package the entire thing and it's ready to be distributed.

- [x] Create necessary files for the application bundle (Info.plist, icons, etc)
- [x] Automate the creation of the application bundle with mac_application_bundle.sh
- [x] Create a disk image (DMG) background
- [x] Create a DMG template
- [x] Create a Makefile to automatically create a DMG based on the template
- [ ] Create documentation about how to build, package and deploy on a mac
- [ ] Test on a non-dev machine

This PR is still Work In Progress, as I haven't finished up the documentation and I haven't tested it on an actual non-dev machine.