readonly attrs overwrite _editor.setReadOnly(true) in onLoad callback

[freshsun]

$attr.$observe('readonly') 
    attrs.$observe('readonly', function (value) {
          acee.setReadOnly(value === 'true');
 });

if readonly is not defined in attributes, it will return false, then overwrite _editor.setReadOnly(true) in onLoad callback in my case

[douglasduteil]

83 Must fix this

Following #74