AMIGAOS: Add AmigaOS version format

AmigaOS features a "version" command which can read out version information if stored in a sepcific way.
To get to that information it parses the exe for "$VER:" and prints out everything behind it.

This adds such a version information to ScummVM so users on AmigaOS can read it out without the need to use "scummvm --version"
Merge pull request #490 from raziel-/patch-2

AMIGAOS: Add AmigaOS version format