Iss 26 create category model and associations

Created models: 
`Category`, with one column: `name`
`CategoryDare` as a join table with: `category_id` and `dare_id`

Added many:many associations for Category:Dare models

Added strong_params for categories to the `dare_params` method in the Dares controller

Modified `views/dares/_form_new.html.slim` to include a checkbox list of all categories in db. This should create the associations automatically on dare creation.

Test in rails console with:
`Dare.last.categories`
`Category.find(id_goes_here).dares`