Automatically insert a download link for the converted file once the
conversions completes.

This uses jquery on the client side.  Two new API methods are
provided:

    <...>.mp3/convert_start
    <...>.mp3/convert_cont/<progress>/<total>

The first starts the conversion and returns an AJAX dict with the
following items:

    "status": "enqueued"
    "progress": 0
    "workload": X

X is the number of megabytes to convert (including both the current
wav file and any previously queued wav files that will be converted
before the current one).

If somebody else converted the file before the request was made, it
may also return:

    "status": "done"
    "size": human-readable size of the mp3 file
    "link": download link

The convert_cont API waits until the progress information is
significantly different than what the client says it last saw, but
always waits at least 0.2 seconds.  Then it returns a JSON-encoded
dictionary:

    "status": "progress".
    "progress": Y
    "workload"": X

X is as in the "enqueued" mapping above, and Y is the progress so far.
Once Y == X, the conversion is complete.  If that has happened,
convert_cont will instead return a mapping with status "done" (see
above).

Redesigned how wav2mp3d reports and keeps track of progress.  The
wav2mp3c demo client is not updated, so it probably no longer works.

Also, raise InternalError if the daemon fails to start quickly enough.

This typically means that the socket file is left, but the daemon
itself is no longer running.  Try to spawn a new one.

By using a simple return statement in the normal case when a wav2mp3d
daemon is already running, the code can be simplified.

On the mirjam setup, BASE is served via CIFS, which
apparently does not support named sockets.

Install it in conf-available, not conf.d.  Use a2enconf to enable it.

Rewrote path handling in using a PathHandler helper class.

Create the mp3 files atomically.

This makes it readable by the www-data user (or whatever
user the web server runs as).

This simplifies the code slightly.

Use a simple form on a separate page so that the file
listing looks nicer (and is valid HTML).

Still very ugly, and there are permission issues that
needs to be solved.

Create the mp3wavweb.conf Apache configuration file.

Also added a toy client that can talk to the daemon, for testing purposes.

This code cannot yet convert to mp3, but works
if an mp3 file already exists.