Make the arguments of os.epoch, time and day case-insensitive

[Luca0208]

I don't see any reasons why the arguments of os.epoch should be case-sensitive.
This could be a problem especially with UTC when people try os.epoch("UTC"), but I think it would also make sense for "local" and "ingame" to be checked case-insensitive.

[SquidDev]

It might be worth doing .toLowerCase(Locale.ROOT) instead (or Locale.ENGLISH) so you don't get strange behaviour on non-English systems. This change should probably be made to os.time and os.day as well (oh goodness, who thought the time changes were a good idea?).

[Luca0208]

Also if there is any reason to keep the arguments case-sensitive I would suggest to at least allow "UTC" and "utc"