Include functionality to submit files to MusicBrainz in addrelease.py plugin

- This patch abstracts functionality previously grouped all into the `callback()` method of the `AddClusterAsRelease` class into a new parent class, `AddObjectAsEntity`.
- It then proceeds to use this new parent class to foster two new classes: `AddFileAsRecording` and `AddFileAsRelease` that allows for adding files as recordings and releases respectively.

I'm not sure about the updated `PLUGIN_NAME`. Suggestions welcome.