Allow more statuses besides 200 as "success"

According to the HTTP statuses list, the whole 2xx range counts as success. Additionally, 304 "not modified" can be counted as success.

See: http://en.wikipedia.org/wiki/List_of_HTTP_status_codes.
See also: https://github.com/chernikovalexey/Prevel/blob/master/Sources/Ajax.js (they allow the same HTTP status range).