[Proposal] Redirect::back() fallback

Added fallback argument to Redirect::back().

For when back doesn't have a referer to send user back.

- Users accessing the link from a bookmark, history or by typing the link manually do not have a referer.
- IE has also been known to remove the referer in situations revolving around javascript. Such as window.open, window.location and even setting target="_blank" in anchors or meta refresh.
- Clicking an embedded link in a chat application, PDF/Word/Excel document, will also not set a referer.
- Using AJAX, file_get_contents, fopen and other similar functions in other languages will probably not set a referer request.
- cURL, fsockopen, applications that have browser-like components might not set a referer.

(From http://stackoverflow.com/questions/5643773/http-referer-not-always-being-passed/5643868#5643868)

Related issue/request: https://github.com/laravel/framework/issues/952