chore(makefile) add vendor task

Makefile

 build:
 	go build
 
+# saves/vendors third-party dependencies to Godeps/_workspace
+# -r flag rewrites import paths to use the vendored path
+# ./... performs operation on all packages in tree
+vendor: godep
+	godep save -r ./...
+
 deps:
 	go get ./...
 
@@ -10,3 +16,6 @@ watch:
 	# for portability, use watchmedo -- pip install watchmedo
 	@watchmedo shell-command --patterns="*.go;" --recursive \
 		--command='make' .
+
+godep:
+	go get github.com/tools/godep