[phobos] getenv, setenv and unsetenv implemented for Windows

[Lars Tandle Kyllingstad]

Today, I noticed that the std.process functions setenv() and unsetenv()
only have POSIX implementations.  So I wrote Windows implementations for
them:

http://github.com/kyllingstad/ltk/blob/master/ltk/system.d

You'll also note a new Windows implementation of getEnv() and a new
function allEnv().

Seeing as this is my first foray into Windows API land ever, I thought
I'd post it here before incorporating anything into Phobos.  Please
comment -- in particular I'm curious about whether it is correct to use
the UTF-16 functions.

-Lars