addrelease.py: Abstract out submission URL.
addrelease.py: Add AddFileAsRecording class.

Allows for easily adding individual files as standalone recordings.
addrelease.py: Add AddFileAsRelease class.

Allows for easily adding single files (which can't be clustered) as releases.
addrelease.py: Update plugin metadata.

- Add Freso to PLUGIN_AUTHOR
- Update PLUGIN_NAME and _DESCRIPTION to describe added file
  functionality.
- Increment PLUGIN_VERSION to 0.5.