Issue28

Fix #28: implement correct semantics for handling the argument of include, include_once, require, require_once.

The docs are unclear but from looking at Zend's source code, it seems that the rule are:
* if the argument starts with `/`, `./`or `../`, resolve the name relative to CWD
* otherwise, resolve it by trying the include_path, then the including file's directory, then the CWD