Set FlushInterval when initializing httputil.ReverseProxy

Due to httputil.ReverseProxy's buffering behavior, I have to wait before HTTP 1.1 Transfer-Encoding: chunked response to start being sent. Setting FlushInterval to httputil.ReverseProxy solves this issue.

Hard-coding the flush interval of 500ms may be arguable.

Note: due to martini-contrib/oauth2's incompatible changes, this branch can't be merged to current master...