[build] Define go.mod (#83)

Removes committed /vendor dir. Retains Godeps (dep) manifests for the time being.

Author: elithrar

go.mod

@@ -0,0 +1,7 @@
+module github.com/gorilla/csrf
+
+require (
+	github.com/gorilla/context v1.1.1
+	github.com/gorilla/securecookie v1.1.1
+	github.com/pkg/errors v0.8.0
+)