Add a linter rule to detect implict method visibility

Summary: I'm not sure whether this is in any coding standards, but it seems to be an unspoken rule that methods should have a visiblity modifier (public, protected or private) explicitly specified. According to the PHP documentation, methods declared without any explicit visibility keyword are defined as public.

Test Plan: Added a test case.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D10687