Use Godep to vendor distribution dependencies

As we get closer to release, we need to ensure that builds are repeatable.
Godep provides a workable solution to managing dependencies in Go to support
this requirement. This commit should be bolstered by updates to documentation
and build configuration.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

Godeps/Readme

@@ -0,0 +1,5 @@
+This directory tree is generated automatically by godep.
+
+Please do not edit.
+
+See https://github.com/tools/godep for more information.