Use a vendor directory for repeatable builds - fixes #816

This is using godep to manage the vendor directory.

vendor/github.com/tsenart/tb/tb.go

@@ -0,0 +1,4 @@
+// Package tb provides a generic lock-free implementation of the
+// Token Bucket algorithm where non-conformity is handled by the user.
+// http://en.wikipedia.org/wiki/Token_bucket
+package tb