datetime review part 2 [Update 5]

[Jonathan M Davis]

Most Recent: http://is.gd/hovyR

Other than some minor documentation changes, all of the changes are to the 
implementation rather than the API, though those changes are fairly minimal as 
well (albeit important).

I fixed it so that the stopwatch code uses a monotonic clock on both Windows and 
Linux (at least, it will once clock_gettime() is fully in druntime rather than 
commented out) and so that getting the current time doesn't use a monotonic 
clock (before Linux was using a normal or real time clock for both whereas 
Windows was using a monotonic clock for both). I also consolidated some of the 
code in static ifs to minimize code duplication.

As a reminder, if you want to use the code on Windows, you need to link with 
advapi32.lib.

- Jonathan M Davis