Use url() to get the correct image path in theme_imagebutton()

This patch adds a url() to make the image path in theme_imagebutton() work wherever you install the module.

With this patch I can see the delete image on admin/settings/voting_actions/edit/* pages. Without it I get a broken image icon.

A big thank you for your work on Voting API and Voting actions Eaton! I have just stared testing Voting actions together with Vote up/down, the version that seems to work for me is DRUPAL-5 from cvs.