[KinoSearch] Weight requires a Similarity

[Father Chrysostomos]

I just got this error message when testing my RegexpTermWeight class:

Error in function kino_Weight_init_from_hash at ../c_src/KinoSearch/ 
Search/Weight.c:24: Can't find 'similarity'

Although this is not documented, Weight apparently requires a  
Similarity object to be passed to its constructor. Is this permanent  
(in which case I can write a doc patch)? or is this going to change?

Note that this also prevents the example in ::Cookbook::WildCardQuery  
from working.