Added all CRUD operations for Categories
Added show and edit links for each category, new category & entries links
Added categories#edit view, extracted form from edit & new into a partial
Added categories#update action; changed controller inheritance to ApplicationController to allow delete actions in links
Added categories#destroy action
Added destroy links in categories#index
Added links to categories#show view
Added back links to categories#edit and new views
Added error messaging to categories new/edit form