CRM-8371 Drush fails all commands with CiviCRM findable and php 5.3.x

With php 5.3.x (current Ubuntu Natty version), you can't run drush against a site that has civicrm in the file system. Drush errors with:

Deprecated function: Assigning the return value of new by reference [error]
is deprecated in _drush_add_commandfiles() (line 941 of
/usr/share/drush/includes/command.inc).

Note civi doesn't even need to be installed. This error is occuring b/c the drush include file uses the (unneeded) php 4 method of assigning new by reference:
$someobj =& new SomeClass();

This hasn't been necessary since PHP 5 came out. Since Civi requires php 5 we can safely fix these.