http.c: new functions for the http API

http.c: new functions for the http API

The new functions added are:
 - http_request() (internal function)
 - http_get_strbuf()
 - http_get_file()
 - http_error()

http_get_strbuf and http_get_file allow respectively to retrieve contents of
an URL to a strbuf or an opened file handle.

http_error prints out an error message containing the URL and the curl error
(in curl_errorstr).

Signed-off-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>