avoid overwriting include path

- fixes pear bug http://pear.php.net/bugs/bug.php?id=17045
- refs e115fc98, de36f63, 51c697a, 9c19304
- solves installation via composer where other pear packaegs are *NOT*
  installed to same include path as PEAR package itself
Merge pull request #32 from glensc/bug-17045

avoid overwriting include path