Introduce Author model

It's really really a rough version.. this enables a authors#show page, to list all quotes belong to that author.

These commits are:

- refine search(auto complete) to reflect quote <=> author relationship
- update sitemap

I have a simple script to migrate the old data to authors table.

```ruby
Quote.all.each do |quote|
  author = quote.old_author
  next unless author.present?
  author = Author.where(name: author).first_or_create
  quote.author = author
  quote.save
end
```

Still tons of TODOs..