mv windows only code into windows only file

related to issue #160
diff --git a/request-stub.go b/request_windows.go
similarity index 74%
rename from request-stub.go
rename to request_windows.go
index b04b991..2ea6be2 100644
--- a/request-stub.go
+++ b/request_windows.go
@@ -1,5 +1,3 @@
-// +build !cgo,!plan9 windows android
-
 package sftp
 
 import "syscall"