Link URI re: issue 486

The proposed changes address the silent failing of adding a link to an unrecognized URI. They allow the user to correct any potential mistakes made to the initial address through a prompt dialogue. 
this check should be centralized, since we perform it in multiple locations (i.e. here and in attachments.js). I would add a method to `Zotero.Attachments` called something like `cleanAttachmentURI`, which takes a possible URI, performs validity check, prepends `http://` if necessary, and returns the result. If the check fails, it should return `false`.