Plugin callback initial code:

• Coded non-blocking media and playlist callbacks, still need to code
  callback for nextplaylist that is blocking.

• Tested with callback function from the vastfmt plugin's script that
  supports the media callback.  Also implemented a playback stub to test
  multiple callback types and functionality.  That was later removed
  from the vastfmt plugin.

Future Work:

• Parse tags from media rather than simply passing the filename

• Cleanup string parsing when we move to C++

• Pass more details in the JSON data for playlist.  Currently we only
  pass each of the sequence entries, without numbering them.  We should
  look into passing not only indexed sequence entries, but other data
  like the "first" and "last" flags for the playlist.
Minor tweaks to the code:

• Only call MediaCallback for media types (not pause, nor events)
• Check for existance of media in JSON data passed to the media callback.