Case insensitive search for words in Phabricator stories

Summary:
When a Phabricator story contains some relevant keywords,
it could be matched to a specific group.

We want to be able to match regardless of the case.

Fixes T773.

Test Plan: A test has been added to address the bug.

Reviewers: dereckson

Subscribers: alken-orin

Maniphest Tasks: T773

Differential Revision: http://devcentral.nasqueron.org/D326