Support the Ferret "=" (exact match) operator in the actual query engine

Summary:
Ref PHI778. In D18492, I added support for parsing this operator, but did not actually implement it in the query engine.

Implementation is fairly straightforward. This supports querying for objects by exact title with title:="exact title". This is probably a bad idea, but sometimes maybe useful anyway.

Test Plan: Queried for title:="xxx", found only exact matches.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: ahoffer2

Differential Revision: https://secure.phabricator.com/D19529