stamp to date-time and the inverse.

This was actually taken directly from pawn's source code. It's one of those functions that aren't included in SA-MP's version of pawn. I converted it's internal function to actual pawn code. It hasn't been cleaned up or anything, just converted to a working state. I also added a gmh (Greenwich Mean Hour) and gmm (Greenwich Mean Minute), both being derivatives of GMT (Greenwich Mean Time).

Converted from a Python script I found and fitted to stamp2datetime's flavor. This function also has not been cleaned up, I have only converted it to a working state.