Search plugin doesn't echo form tag and has its width hard-coded.

The search plugin doesn't echo the form_tag, which was deprecated in Rails 3.0 and unsupported in Rails 3.1. This fixes that.

Also, the width of the search bar is hard-coded, which makes it hard to incorporate into custom themed sidebars. This patch moves away from the hard-coded size to a pixel width that is configurable on the sidebar customization page.