Added respond_to_missing? implementation to ThinkingSphinx::Search

... so it correctly says that it responds to methods delegated to masks.

It solves a problem with duck-typing; for example, if you have some code which tries to detect if your object represents a paginated collection by checking ```respond_to? :total_entries``` result, then it won't realize that ThinkingSphinx::Search object is, in fact, paginated.