Use repo-relative urls for screenshots

Former screenshot urls point to images on wordpress.org, but end up in 404 error. Once screenshot images are in the repo, just show those images via relative urls.
Added italian translation
Handle constant values as constants
New boot up flow

Removed any static-ish boot flow. Current instance can be accessed via
filter. Hooks addition is moved from constructor to ad-hoc enable()
function, allowing hooks removal via disable() method.
Removed setup_admin_hooks()

setup_admin_hooks() was reduntant, because is was just a wrapper for
metabox_script() that, anyway, was already called on
'admin_enqueue_scripts'.
Remove from MetaBox CPTs that should be removed

CPTs having show_in_nav_menus set to false or that aren't publicly
queryable shouldn't be added to metabox. However, the
"show_{$ptid}_archive_in_nav_menus" filter hook can be used to force
them to be added.
Do nothing if no archiveable CPTs registered

Not all WP installs have CPTs registered. And not all CPTs should be
shown even if registered and having archive (see
b292680a9fa8d9b3648855b166c8b2c72fd7566b). If there are no valid CPTs,
plugin have nothing to do on backend (on frontend already added archive
links should continue to work)
V 1.3 added @giuseppe-mazzapica to contributors
Backward compatibility for removed object vars
Avoid $_POST direct access on ajax
Updated language files
Echo 'No items.' rather than return if no CPTs

Core shows 'No items.' when menu items are not available to be added.
Use same approach.
Composer support
Extended .gitignore
Fixed stupid things on composer.json
Update readmes to reflect behavior when no CPTs
Merge pull request #26 from Giuseppe-Mazzapica/GMv1_3

Merge v1.3 PR. New boot up, improved "archivable CPT" selection, composer support, prints 'no items' when none CPTs found.