Improve message on exception raised by Maestro.set_data()
Remember everybody that requested a given package
Improvements for the Installer command

 * Update the installer to use the new Maestro API;
 * Moved the UI-ish code to the curdling.tool namespace;
 * Increased the test coverage;
Remove quotes from the Content-Disposition in the downloader
Service().{queue,handle} do not receive `requirement` anymore.
Change services.Dependencer() so it will read the requirement from `data`
Break down the downloader into Finder and Downloader

The Downloader service was broken down in two separate services. The
Finder, that only looks up package indexes and the downloader, that
takes any (known) URL and downloads it;

Both `{Finder,Downloader}.handle()` methods are compliant with the new
`Service.handle()` signature.
Update tests to leverage Downloader/Finder update
Maestro: Add statuses FOUND, CHECKED and locator_url, url vars
Add Service.__call__(requester, **data) as alias to Service.handle()