Add new feature: Private comments and personal note

action.php

backup/moodle2/backup_pdfannotator_stepslib.php

classes/output/comment.php

classes/output/index.php

controller.php

db/access.php

db/install.xml

db/upgrade.php

lang/en/pdfannotator.php

locallib.php

mod_form.php

model/comment.class.php

model/pdfannotator.php

model/statistics.class.php

settings.php

shared/index.js

shared/locallib.js

shared/statistic.js

styles.css

templates/index.mustache