added blog ER diagram, also added has_many and belongs_to associations for author and post
added has_many and belongs_to associations between comments and posts.
added PostCategories join model and table to the database. created associations in the post and category models to reflect the conceptual many-to-many relationship between posts and categories. still need to add user_id to post and post_id to comments.
added author association of the author and post and made sure that the post params passed the author_id and not the author object.