When asking for the API key, inform the user to sign up if they don't have a WP Remote account.
To make developing locally easier, add a helper function for getting the wpremote.* URL

Addresses https://github.com/humanmade/WP-Remote-WordPress-Plugin/pull/22#discussion_r4109538
Merge pull request #22 from humanmade/prompt-signup

Link to signup flow from "Enter API Key" prompt