Rename struct Encoder to PreparedEncoder, and add a new (abstract)
class Encoder which represents one encoder instance.

.. and move the plugins to src/encoder/plugins/.

Replaces GLib's GError.

This is necessary for Ogg packets that span more than one page.

Make sure the file header gets written at the beginning, before
_write() gets called.

Add the encoder_plugin method end().  This is important for the
recorder plugin.

In the "vorbis" plugin, this is a copy of the old flush() method,
while flush() gets a lot of code remove, it just sets the "flush" flag
and nothing else.  It doesn't start a new stream now, which should fix
a few problems in some players.

These fixes were mostly generated with `codespell' [0] and manually
reviewed.

[0] http://git.profusion.mobi/cgit.cgi/lucas/codespell/

This updates the copyright header to all be the same, which is
pretty much an update of where to mail request for a copy of the GPL
and the years of the MPD project. This also puts all committers under
'The Music Player Project' umbrella. These entries should go
individually in the AUTHORS file, for consistancy.

The new generic encoder API will replace shout's custom encoder API.