Prepare for dependency vendoring.

- Add helper script to simplify glide usage.
- Add validation script for unwanted changes to vendoring.
- Relax/tighten up .{git,docker}ignore to cover vendored files properly.
- .validate: Protect from unbound variable in case of nounset setting.
- Install more recent hg version in the build container.
- Remove glide installation steps from Dockerfile.
- Update documentation.

script/.validate

@@ -1,6 +1,6 @@
 #!/bin/bash
 
-if [ -z "$VALIDATE_UPSTREAM" ]; then
+if [ -z "${VALIDATE_UPSTREAM:-}" ]; then
 	# this is kind of an expensive check, so let's not do this twice if we
 	# are running more than one validate bundlescript