stdbin: add pwd #243

why?

Because cygwin's pwd sucks on windows:

λ> cd C:\Users\Tiago\go\src\github.com\NeowayLabs\nash\stdbin\pwd
λ> C:\tools\cygwin\bin\pwd.exe
/cygdrive/c/Users/Tiago/go/src/github.com/NeowayLabs/nash/stdbin/pwd

why?

Because of that several tests fail on windows when using cygwin tools....
I propose a simple pwd that returns sane output on every architecture, not imposing unixys on every one.