make dns resolution work in weave container

by using go's internal DNS resolver.

NB: this relies on go's std library having been compiled with the
netgo build tag, e.g.

   sudo go install -tags netgo -a std

issue #93

weaver/Makefile

@@ -6,8 +6,8 @@ SUDO=sudo
 all: docker-image
 
 weaver: ../*.go main.go
-	go get
-	go build -ldflags '-extldflags "-static"'
+	go get -tags netgo
+	go build -ldflags '-extldflags "-static"' -tags netgo
 
 .PHONY: docker-image
 docker-image: /var/tmp/weave.tar