Add ability for the moodle plugin to use a public or LTI connections.

* Refactor the plugin structure to match moodle 2.3 conventions
* Centralize function in local/mediacore/locallib.php
* Add ability for lti-type connections
* Create mediacore_media and mediacore_media_row class
* Update the filter to allow LTI type embeds
Update the README.md
Add updated screenshot