Simplify ignoring case in lexer

Summary:
I like the expressiveness of regexps.
Plus almost everything is case insensitive in PHP, it was hard to find something which is not for the test case.

Also added more case insensitive checks and included 'S' flag mentioned at D3336#7.
Also used identifier pattern in function definition.

Test Plan: New test case.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3366