Various 0.6 bug-fixes

This branch fixes a bunch of issues:

* Fixes description for HTTP runner parameters
* Add description for proxy HTTP runner argument and hook it up so it actually works
* Add description for `redirects` parameter and rename it to `allow_redirects` so it's more explicit and obvious
* Add description for `cookies` HTTP runner parameter and fix the type to `object` so it actually works
* Fix an issue with runner type registration - previously, we would never update the objects in the DB which means that if we updated runner definition this would never get reflect in the DB unless the user manually cleared the runner_type_db collection.
Could be if schema.get('default', None)...