Add missing foreign keys and missing relations in models

* Also remove ip_address fields - we have no business collecting that
* Add comments to a posts user_id/type
* Tasks to posts/users/parent
  * ON DELETE SET NULL for assignee/assignor - just unassign
  * ON DELETE CASCADE for post or parent
* post_* to form_attributes
  * ON DELETE SET NULL - to avoid mass deleting data if attribute removed
* post_comments to users / parent
  * ON DELETE SET NULL since a comment can just be treated as anonymous
    once a user is delete. Top level if parent deleted
* posts to form / users / parent
  * All using ON DELETE SET NULL to avoid mass deleting data when a form
    is remove
* Relations
  * Add form_attribute relation for post_*
  * Add form relation to post
  * Add posts relation to form
Merge pull request #37 from ushahidi/missing-foreign-keys

Add missing foreign keys and missing relations in models