added recursive delete function to io.php
added tests for io_rmdir
Merge branch 'recursivedelete' into extension_manager

* recursivedelete:
  added tests for io_rmdir
  added recursive delete function to io.php
now use new core funtion to recursively delete
check for admin in AJAX backend
purge cache only once on install

this is not a extension specific cache but a global one. no need to
purge for each installed extension
typo fix