Separate tracker logic from the http package, step 1
Update tests and fix a couple failures
Move NewAnnounce and NewScrape to http, since they are an implementation detail
Put the query package into http, since it is for http only
Move AnnounceResponse and ScrapeResponse to models
Clean up types more
Move backend to root level, combine announce tests, rename files
Remove a small file, and add comments
Add new option to the example config, and update README