Add a filter to exclude specialist_sector documents

This modifies UnifiedSearchBuilder, adding a new filter to all search 
queries that excludes documents in the specialist_sector format.
Include the sectors filter in test query_options

This is a first pass at trying to fix tests broken by the inclusion of
the new filter, by including the specialist sectors filter in the 
default query used by most tests when building stubs.
Fix UnifiedSearcherTest
Fix UnifiedSearchBuilderTest
Merge pull request #253 from alphagov/register-specialist-sectors-in-rummager

Exclude specialist sector documents from unified search results