Improve docs a bit.
plugins.gomtv: Fix for latest VOD player changes.

Newer VODs are now also using the key check server.
Rearrange a few things.

- Clean up all the package __init__.py files
  and move code inside them into separate files.
- Add metadata to main __init__.py file.
- Move exceptions into a separate file.
- Let setup.py fetch version from source instead.
Update plugins with new imports.