Rank search results by word count.

Fixes #436. Add an option that tells the indexer to compute word
count into an attribute, and change the ranking formula to rank
results by inversed word count.
Avoid potential division by zero.

Refs #436.
Added index_field_lengths in cjk_common_index.
Merge pull request #451 from Tatoeba/length-ranking

Rank by word count