Remove mediacoreinsert. Add new mediacore tinymce plugin.

* Displays dropdown of public and mapped LTI connections
Remove mediacorefilter. Update to new filter/mediacore plugin.

* Update the filter to allow LTI type embeds
Add local/mediacore libraries.

* Refactor the plugin structure to match moodle 2.2+ conventions
* Centralize functions/classes in lib.php and locallib.php
* Create mediacore_media and mediacore_media_row class
* Add ability for lti-type connections
Remove old readme.txt file
Add new README.md
Add updated screenshot