Lots of changes

Remove functions pinched from emms-extension.el.
Rename emms-tag-mark-regexp to emms-mark-mark-tagged.
Create new commands: emms-mark-mark-artist, emms-mark-mark-composer,
emms-mark-mark-performer, emms-mark-mark-title, emms-mark-mark-album,
emms-mark-mark-year
Rename emms-ext.el to emms-mark-ext.el
Update documentation and add autoloads
Initial commit of Readme.org