Allow plus signs in URL's to pass unscathed in Helper functions.

According to  RFC 1738 the plus sign does not have special meaning outisde of the query part of a URL.
Router::url should passthru //example.com/file.ext

The function allows ://example.com/file.ext but was treating //example.com as cake-relative URL. The updated regex matches URI schemes as defined in RFC2396. Will passthru any of these formats:
* Starts with a valid URI scheme  (javascript:, https:, itunes:, ftp:)
* Starts with a '#'
* [NEW] Starts with a '?' which may be meaningless, but is as valid as starting with '#' (RFC1808)
* starts with //, or :// (:// is not technically valid, but included for compatibilty)
Fix subtle error in Regex matching URI scheme.

[+-.] will match plus, hyphen, and dot, but also comma. Use [+\-.] since  commas not allowed in URI scheme.