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. 
I'm not sure privileging ```http://``` actually makes sense, both because non-HTTPS connections should sort of be viewed as deprecated in 2014 and because it's weird for something like "asfasfasfag" to turn into "http://asfasfasfag". The latter could be addressed by at least checking for a period (and if someone really wants a local server, they can add the scheme), but I think we should actually just require a proper URI.