Add search functionality

I've implemented the search functionality, it should fix #36

Basically, it will fetch package list from npm and simply append the local ones in the response.

This patch:
* add a new route to match the search queries (`/-/all/*`).
* add a new `get_recent_packages` method in `local-storage` to list the packages more recent than a given timestamp.
* add a new `search` method in `storage` to run the request to npm and to append the local packages (found with `get_recent_packages`) to the response.