1 second cache prevents a latest post from publishing

A 1-second cache was added to fix #30 but publishing posts in the admin interface leaves it missing.

A workaround is to simply edit a draft and save it, causing a new site generation.

The problem at its core is:

```ruby
if params[:publish] == "yes"
  content.publish!
end

site.generate
```

The site's posts are changed and then `site.generate` is called, which uses the stale list.

Perhaps `Site#generate` can force the cache to invalidate. Since it's the processing time _after_ generation that we want to avoid being slow, and `to_liquid` should be in use for generating content, wiping it out at the beginning of a generation should be fine.