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.

.gitignore

@@ -1,15 +1,13 @@
 /dist
-gen.go
+/autogen/gen.go
 .idea
 .intellij
 *.iml
-traefik
-traefik.toml
-*.test
-vendor/
-static/
+/traefik
+/traefik.toml
+/static/
 .vscode/
-site/
+/site/
 *.log
 *.exe
 .DS_Store