Paired down OS Lua library

Implements the following functions:

• os.clock
• os.time
• os.date
• os.difftime

Pretty much removed everything except for time operations. File operations steps on the toes of the IO library, and the others would be either invasive or pointless.

The main reason I want this is because os.time provides an OK way to initially seed RNG. But it would also be cute to let Lua modders be able to implement Christmas mode or day & night cycles themselves :p

See also: http://lua-users.org/wiki/OsLibraryTutorial