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 RegExp would fail on something like `zotero.org/download`, since the colon is not part of the negative lookahead. But you can also do something like
```
input = input.trim();
var protocol = input.match(/^([a-z][-+.a-z]*):(\/\/)?/i);
if(!protocol) {
  // there's nothing here resembling a protocol, so prepend "http://" and make it a URL
} else if(listOfAllowedProtocols.indexOf(protocol[1].toLowerCase()) == -1) {
  // not an allowed protocol, ask user to correct
  // note that this could be for something like "localhost:8080/foo/bar", which, while could be
  // a valid URL is we prepended "http://", the user would have to be explicit about what protocol to use
} else if(['http', 'https', 'ftp'].indexOf(protocol[1].toLowerCase()) != -1 && !protocol[2]) {
  // missing "//" part, which is required for these protocols. Do not allow
}
// at this point, the URI should be OK
```