Extracting attachments from links and image sources.
Added WP Importer as a vendor dependency.
Attachment URL mapping.
Added composer install to Travis build.
Add an empty post_title for attachment.

Cf. [wordpress-core@24759](https://core.trac.wordpress.org/changeset/24759)
Upgrade notice for the removal of PHP 5.2
Add tests for import->saveMedias()
Merge pull request #3 from oncletom/import-attachments

Improved Media Import