Add an XHPAST::getNamespace() method

Summary: This throws away D14498 and starts again. Basically, the grammar of PHP does not allow namespace statements to be parsed consistently (see some discussion in D14506). Instead of complicating the grammar, move the logic of determining a node's namespace to the PHP representation of the AST.

Test Plan: Added unit tests.

Reviewers: richardvanvelzen, #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14512