sheep/http: add range support for swift insterface

In swift spec, "Range: bytes=13-15" in http header means "read the three bytes
of data after a 13-byte offset". We add this support by parsing http header and
read specific bytes in range.

We don't support multi-ranges at present.

Signed-off-by: Robin Dong <sanbai@taobao.com>
[ return Range error instead of system err for out of range get - Yuan ]
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
test: add test case for RANGE GET in swift interface

Signed-off-by: Robin Dong <sanbai@taobao.com>
[ adopt to new error code for out of range get - Yuan ]
Signed-off-by: Liu Yuan <namei.unix@gmail.com>