allow custom classes in custom locations

Only add class path using app::uses when class is not available.

So I can App::uses('CustomFilter', 'Lib'); in bootstrap.php without it being overwritten by the _buildFilters method.
Merge branch 'master' into custom-classes

* master:
  Dont use deprecated method
  Update changelog for 0.15
  Make AssetScanner::find() able to return relative paths.
Merge pull request #210 from joostdekeijzer/custom-classes

allow custom filter classes in custom locations