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

Resolves #95 
Resolves #123 

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
Just to get the update code into a pattern, so for future updates that contain more than one set of upgrade actions, e.g.

```php
if ( version_compare( $current_version, '0.6.2', '<' ) ) {
    // update s3 meta with bucket region where missing
    $this->update_meta_with_region();
}

if ( version_compare( $current_version, '0.8', '<' ) ) {
    // update something
    $this->update_something();
    // do something else
    $this->do_something();
}