Test and refactor Event#search.

Test and refactor SQL search engine implementation of Event#search. This lays the groundwork for eventual object extraction.
Yeah, that thought came to my mind, too, but I wasn't sure if it would be more or less clear. What do you think?
```ruby
conditions = query.scan(/\w+/).inject([[]]) do |conditions, keyword|
  like = "%#{keyword.downcase}%"
  conditions.first.push 'LOWER(events.title) LIKE ? OR LOWER(events.description) LIKE ? OR LOWER(events.url) LIKE ? OR LOWER(tags.name) = ?'
  conditions + [like, like, like, keyword]
end
conditions[0] = conditions.first.join(" OR ")
scope = scope.where(conditions)
```