Range support for attachments in trunk

[Robert Newson <ro...@gmail.com>]

All,

My patch to support the Range header is now on trunk. Attachments are
upgraded for efficient seeking on compaction and do not support the
Range header until that happens. "Accept-Ranges: none" is returned for
non seekable attachments and "Accept-Ranges: bytes" is returned for
the new style ones.

I inspect #att.data in couch_http_db which I've marked as a layering
violation. I've also modified couch_stream.erl and couch_file.erl so
it needs careful review.

B.