Adapted OAS to use usage event plugin

Can you please test those modifications? I was able to test as far as I could, but without the OA-S keys I couldn't test everything. All I know is that the plugin is correctly receiving the usage event, but that's it.

To test it you will also have to pull the usage event plugin commit: https://github.com/pkp/ojs/commit/ab93ebc4b3f49352d8b52dbd19e34c1d1a21444b

If you have any problem testing this stuff just tell me that I can fix it. Thanks. :)