Request to join group

This pull request addresses #458
So not sure what is here the best, just throwing ideas out.

Maybe we should do this through a separate publish, which would publish a group field of pending clients, but would be filtered only to that user. So:

```
Meteor.publish 'my-pending-group, (groupId) ->
  @related (person) ->
    return unless person?._id

    Group.documents.find Group.requireReadAccessSelector(person,
      'pendingMembers._id': person._id
    ),
      fields: _.extend Group.PUBLISH_FIELDS().fields,
        'pendingMembers.$': 1
  ,
    Person.documents.find
      _id: @personId
    ,
      fields: Publication.readAccessPersonFields()
```