No gettimeofday on Windows

[headius]

For jruby/jruby#4393 we added gettimeofday to jnr-posix. Unfortunately, it is not supported on Windows (jruby/jruby#4565).

I'm doing a workaround that uses System.currentTimeMillis on Windows. This won't preserve microsecond accuracy, but it will work.

We need to do the additional logic to make an appropriate gettimeofday on Windows. I found at least one SO post describing how: http://stackoverflow.com/questions/1676036/what-should-i-use-to-replace-gettimeofday-on-windows