Touchups.

 - Gave `chunk` a default value.  It is useful for debugging and calling
	delete.  Hopefully the fact that it is `0` will be enough to discourage
	anyone from acutually using it in production.
 - Short circuited requests for zero or negative numbers of IDs, they no longer
	fetch no values from a shard but just return an appropriate value for
	no keys.
 - Fixed `shards` initilization.
Doc formatting.