Posts

Create a `posts` resource as a `group` member resource.

 - [x] GET posts#show
 - [x] POST posts#create
 - [x] PUT posts#update
 - [x] Make `Post` a `taggable` resource
 - [x] Give `Post` a `friendly_id`: https://github.com/norman/friendly_id

posts table:

 - id
 - group_id
 - author_id
 - editor_id
 - slug
 - tags
 - title
 - content
 - published_at