Curdler should forward the parameter `requirement`
Ensure that {Finder,Downloader}.handle() forward the requirement param
Retrieve & build process works again after refactoring Install

All transitions between services are now tested, so we can quickly fix
any weird errors that happen in the flow of a requirement;

The method `request_install()` was replaced by `feed()`, that does the
same thing but was rewritten under TDD;

This version does not check if the received requirement is already
installed. It will be reimplement it soon with proper tests;

Currently, the main command ends up in an infinite loop, so it should be
ran with `-q -l DEBUG` so you can see when you can Ctrl-C.
Do not fail when overriding keys in Maestro.set_data()
Polish overall Retrieve-And-Build process

 * The main loop now ends exactly when it should;
 * Install.feed() now filters repeated requirements;
 * The error report is partially working again;