Validate that a backend_url is a valid http url.

We only support plain http backends.
Add validation of a route's backend_id

Validate that it maps to an existing backend
Validate a route's incoming_path

Must be a full URL path
Improve validation of backend_urls

It was allowing through some technically valid, but unwanted values.
Simplify backend_url and incoming_path validation
Merge pull request #15 from alphagov/url_validation

Validate URL parameters