Show suggestion on the 404 page
use slug
add tests for helpful 404 page
separate getting suggestion logic from error handler
handle fallbacks in URL dispatcher
remove unused import
Merge branch 'master' into feature/rich404

Conflicts:
	readthedocs/templates/404.html
load core_tag
fix doc_url tag to return a valid URL
add tests for doc_url tag
doc_url tag supports singlehtml
Huge tasks refactor.
Small fixes.
Also kinda works.
More task refractoring.
Something that almost works.
Save build results correctly.
Refactor builders.

They now simply create the files inside of the projects directory inside the artifacts directory. This allows an outside process to move the files that they generate onto the app servers.
Always set build to finished
Refactor builders and tasks to use proper file placement.

Also put all Sphinx builders in one file.