Fixed for Roundcube 1.0-RC

1. Fix chbox.php
- $rcmail = rcmail::get_instance();
+ $rcmail = rcube::get_instance();
---------------------------------------------
2. Fix chbox.js
There's a bug with the list of options in mail page. You need to add a line in 
 +<skin>/templates/mail.html, if not, when the user saves preferences, the chbox 
 +column will be hidden.
FIX IT:
 $(document).ready(function(){
    chbox_menu();
 -  var li = '<li><input type="checkbox" name="list_col[]" value="chbox" id="cols_chbox" /><label for="cols_chbox">'+rcmail.get_label('chbox.chbox')+'</label></li>';
 +  var li = '<label><input type="checkbox" name="list_col[]" value="chbox" id="cols_chbox" /><span>'+rcmail.get_label('chbox.chbox')+'</span></label>';
    $("#listmenu fieldset ul input#cols_threads").parent().after(li);
 +  $("#listoptions fieldset:first-child ul.proplist li:first-child").after('<li>'+li+'</li>');
  });
---------------------------------------------
3. add images for skins
---------------------------------------------
4. Rename skins\default to skins\classic
---------------------------------------------
5. Add composer.json