Update for compatibility with Omeka

[timstallmann]

Hi! I'm so glad I found this plugin -- just a small PR to:

• Change the plugin.php file to be compatible with modern Omeka plugin syntax
• Upgrade dependencies
• Add a new isObjectAvailable method, which mirrors the method in the Zend_S3 plugin (to check if an object exists).

[mikesname]

@timstallmann thanks! Does the composer.json file also need to be updated? Also, would you mind bumping the plugin version to something like 1.1.0 in the plugin.ini file in the patch?

[timstallmann]

Version number changed! And composer.json is fine as is, just the composer.lock needed to be changed -- aws-sdk-php is still on major version 3.

[mikesname]

@timstallmann I'm afraid I merged this a bit hastily yesterday and missed something.

Will it inconvenience you if I change the name of the class from OmekaAmazonS3StorageAdapterPlugin to just AmazonS3StorageAdapterPlugin? This is most plugins I'm aware of do not have the prefix, it's not an Omeka internal class, and it would also require us to reinstall rather than upgrade our many existing instances.

Sorry about that!

[timstallmann]

For sure, no problem, sorry about that!