Create a swift/auth v2 external files app

This changeset adds support for Swift external storage with v2 (keystone) authentication.

The ``swift_authv2.php`` file is an updated version of the original ``swift.php`` using php-opencloud

Requires php-opencloud in 3rdparty. (cf. owncloud/3rdparty#40)

Also fix .gitignore to handle core apps correctly.