[Issue #200] Adds SEO-friendly IDs to questions and categories

Adds friendly_id for SEO-friendly lookups of Questions and Tags. 

Instead of `/answers/1` we now have `/answers/how-do-i-register-a-speed-boat`
Instead of `/tags/1` we now have `/tags/speed-boats`
and of course,
`/search/tag?tag=speed-boats` instead of `/search/tag?tag=1`

** Do not merge ** 
In order to test, I updated the installer's reference to point to this answers branch.
`gem 'answers', github: '18F/answers', branch: 'feature/200-friendly_ids'`
Prior to merge, this line needs to be adjusted: https://github.com/18F/answers/blob/feature/200-friendly_ids/templates/answers/installer.rb#L18