Retrospective cron batch jobs to update s3 attachment meta with bucket region

Resolves #95 

This needs to be tested with a branch of AWS core that has https://github.com/deliciousbrains/wp-amazon-web-services/pull/28 and https://github.com/deliciousbrains/wp-amazon-web-services/pull/30 merged

This PR introduces a plugin upgrade mechanism where we can do migrations/data updates for new versions. The first being for #95. 

I have set up some general handling of cron events so we can re use in the future - possibly for #44
Why include strings in your prepared statement args? `'amazonS3_info'` and `'%"region"%'` should be just part of the SQL.

I think it's safe to assume any posts that have the post meta 'amazonS3_info' is an attachment, so I think this can be reduced to:

```php
$sql = $wpdb->prepare(
	"SELECT `post_id` as `ID`, `meta_value` AS 's3object'
	FROM `{$prefix}postmeta`
	WHERE `meta_key` = 'amazonS3_info'
	AND `post_type` = 'attachment'
	AND  `post_id` > %d
	AND `meta_value` NOT LIKE '%region%'
	ORDER BY `post_id`",
	$last_post_id
);
```