Include shortcode for notification counter and extend [pm_user] with content

Hey, two suggestions here

1. Include a shortcode so we can build our own notification bubble / button outside of the admin bar

2. Extend the shortcode [pm_user] so it not only allows to prepopulate the subject, but also the content of the message - this could be useful for message templates. For instance we are building a user to user rental system and it would come in very handy if we could pass the shopping cart on to the message body.