lua: remove ufs usage (#78) and fix premake file

[ljrk0]

This removes the usage of ufs and ex get_externals.lua (fixes #78). Changes:

• remove _FindFileInPath.lua because it's not really needed. If one'd care about this, one can always check the return value of os.execute()
• require LuaFileSystem lfs
• update the link to the glSDK (and update it from 0.4.2 to 0.5.2)
• use 7z instead of 7z.exe as binary name, I've no Windows to test on anyway
• implement a RecursiveDelete() function as replacement to ufs.remove_all()
• rewrite RecursiveCopy() to only use LFS and IO
• use UNIX style paths
• use os.execute() instead of ex.spawn() since the latter segfaulted and can be considered deprecated (lua51 only)
• also fixing the premake SetupFrameworkProj() function

This also makes the script compatible to lua5.3