ajax command attr

Command to implement the jQuery attr functionality which will set a single property to a value, on all matched elements.

Example code:

$output[] = ctools_ajax_command_attr(
  'input[name*="[title]"]',
  'value', 
  'New value to set'
);

Will create a ctools ajax command to change the value of every input field with title in its name to "New value to set"

Patch rolled against head.
Comments and suggestions welcome.