Attachment() namedtuple, encode_attachement()

[Jason Gerard DeRose]

namedtuple are pretty darn handy, so I'm adding an Attachment(content_type, data) namedtuple.

CouchBase.get_att() will now return an Attachment rather than a vanilla tuple. This doesn't break the API as the namedtuple works just like a regular tuple when used like one. But the `content_type` and `data` attributes are nice.

I'm also adding an encode_attachment() function as I have equivalent code in Dmedia in a few places and it makes sense to consolidate this is Microfiber.

The function sig is:

  encode_attachment(attachment)

Where *attachment* is an Attachment namedtuple. I have somewhat mixed feelings about this, but I like the fact that it makes round-tripping easy without intermediate code having to unpack the (content_type, data) all the time, or know any details about the attachment data structure. The equivalent function in Dmedia is currently:

  encode_attachment(content_type, data)

Lastly, I'm adding an experimental CouchBase.put_att2() method with an API change I'd like to make. This would be a breaking API change, so I'm adding this temp method to make it easier for everyone to evaluate it before we make a decision. I'm not sure how much put_att() is being used outside of Novacut, so if you're using it, please voice your opinion on this proposed change! The 2 signatures are:

  CouchBase.put_att(content_type, data, *parts, **options)

  CouchBase.put_att2(attachment, *parts, **options)

The rational is similar as for encode_attachment(): you can round-trip the attachment while treating the Attachment namedtuple as an opaque data structure. There are also some future plans I'm considering for using the Attachment namedtuple as a way to make the core REST adapter a bit more generic, although that could be done without changing put_att().

To clarify, this bug itself involves *no* API breakage. If it seems like a good idea, CouchBase.put_att() will be changed in a separate merge. Regardless, CouchBase.put_att2() will be removed after a decision is made.