Fix up links for posts
Add migration and model for category relationships

* Add migration and model
* Category relationship belongs_to post and category
Add category relationships to posts and categories

* Add has_many category relationships to posts and categories models
* Add has_many categories/posts through category relationships
Add category checkboxes to post form

* Pass in categories through new method and new show
Add checkboxes for categories and save

* Add checkboxes for categories to post form
* Add category ids to post params
Add categories to post partial

* Add list of categories to post partial
* Link each category to corresponding category page
Add categories to edit method and view

* Define categories in post edit method
* Pass in categories in post edit view
Refactor posts controller

* Remove categories instance variable and edit views accordingly
* Put :show into array
Merge pull request #18 from elijahkim/ha-add-categories

Add categories to posts