Added `:params` support to `custom_filters_score` query

Example:

    query do
      custom_filters_score do
        query { string 'foo' }

        params now: Time.now.to_i * 1000

        filter do
          filter 'exists', field: 'created_at'
          script "((0.08 / ((3.16*pow(10,-11)) * abs(now - doc['created_at'].date.getMillis()) + 0.05)))"
        end
      end
    end

Thanks to @thor27 and Jon Tai's blog post: http://jontai.me/blog/2013/01/advanced-scoring-in-elasticsearch/

Closes #19