Some love on the Service constructor
New `Index` for managing a directory with packages

This new index unifies the `LocalCache` and DirectoryStorage` features
and it's pretty simple to use. Each package can have more than one file
and there's a very basic search with filters implemented right now;

The services Curdling, Downloader and Installer were updated to use this
new API. The Downloader service also raises exceptions when can't find
the requested package;

A bunch of tests were improved, others removed. Let's keep walking
towards the 100% coverage :)
Starting to separate old code in a different file. They will die soon
Add gevent to the requirements file
Running only unit and functional tests, not running the old ones too