Utilities: Updated addURLparameter to merge the new query string with the old query string.

This should fix a problem where one can end up with multiple parameters with
the same name in the query string. This patch also changes the $parameter of
addURLparameter to be an associative array. For backwards compatibility it can
still be a string.


git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@739 44740490-163a-0410-bde0-09ae8108e29a