Add helper regarding the request URL

[ioppermann]

For routing a request, the URL without the query string is usually required. The method http_request_path() returns the path component of the URL (i.e. everything befor the first "?") and the method http_request_querystring() returns the query string components of the URL (i.e. everything after the first "?").

The method http_request_query() returns the value of a given key in the query. The method http_request_iterate_query() iterates over all query keys.