improve phing composer and relationship

Allow to load composer autoload.php file when phing is executed in a global scope
Allow to place composer autoload.php file in a different folder setted by config->vendor-dir
Allow to ignore errors when taskdef require a class and autoload.php do not exist, in example when a phing task is executed by composer pre-install-cmd.

my environment can not pass all the tests, there are a lot of dependencies and I don't know all. Can you test https://github.com/corretgecom/phing.git before to merge the pull?

thanks