Support b'binary string' in XHPASTNode

Summary: Documented at [[ http://www.php.net/manual/en/language.types.type-juggling.php#language.types.typecasting | Type Casting ]].

Test Plan: New unit test.

Reviewers: epriestley, vinodsr

Reviewed By: epriestley

CC: Korvin, epriestley, aran, vinodsr

Differential Revision: https://secure.phabricator.com/D7662