Split status template into two templates.

This is now using a base template (_base.html) for the 'layout' of the web UI.
Within that base template, the actual content templates get rendered.
Moved graph and console to use templates as well.
Add navigation menu to all pages + change routes.

To achieve that, this PR
- converts static/index.html ("console") and graph to templates
- moved the handlebars template to separated file to avoid escaping issues

Route changes:
/status -> /
/static -> /console
/static/graph.html -> /graph
Remove cruft.
Replace YieldExporter() with Handler()
Fix typo.
Use client_golang.exp for automatic telemetry.
Use Sprintf instead of string concatenation.
Merge branch 'master' of github.com:prometheus/prometheus into feature/navigation

Conflicts:
	web/web.go
Merge pull request #118 from prometheus/feature/navigation

Add navigation bar to web ui