Add Plek#website_uri and Plek#asset_uri methods

This makes it easier to access the website hostname
(Plek.new.website_uri.host) and also to build urls
(Plek.new.website_uri + 'some/path') without having to worry about
things like trailing slashes, etc.
Merge pull request #19 from alphagov/add-website-hostname

Add Plek#website_uri and Plek#asset_uri methods