Adding some seo friendly urls

Does two things:

- Adds `semantic-ui-themes` and `semantic-ui-snippets` to the URLs
- Overrides `to_param` in `Theme` and `Snippet` so their URLs now include more about them. Examples:
  - Theme: `/semantic-ui-themes/1-theme-name`
  - Snippet: `/semantic-ui-snippest/1-snippet-title`

Should close #60 @wallawe