CBLT-42:  add a windows BAT file to do same thing as build_sync_gateway.sh

Even with a cygwin BASH environment, cannot get GO to handle GOROOT correctly.

Change-Id: Ic5b936277dcd14496ca31564a4221f66d702833f
Reviewed-on: http://review.couchbase.org/37092
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>