ClassMemberListNode::setStatic()

This patch implements the following changes, with tests:

- Adds ClassMemberListNode::isStatic(), which returns a boolean indicating if the member list is static (i.e., has a T_STATIC token at $this->static).
- Adds a ClassMemberListNode::setStatic() method, accepting a boolean indicating if the list should be made static or non-static.

Right now, setStatic() assumes there's a visibility in the member list -- wasn't sure how to handle it if there isn't (or if that's even supported by PHP -- something like static var $foo).