Improve UI and structure

- Use Bootstrap for UI
- Use Slim for templating
Remove `show` action
Move custom action to bottom
Remove JSON support (unused)
Add validation to User

- Validate presence of name
- Validate presence of email
Redirect to #index instead of #show
Cleanup UsersController
Merge pull request #9 from KevinSjoberg/feature/improve-structure-and-ui

Feature/improve structure and ui