COUCHDB-769: Store attachments in the external storage.

[gilv]

Initial implementation that allows CouchDB to store attachments outside of the database file. This implementation supports OpenStack Swift and SoftLayer Object store

[kxepal]

Please fix style: 80 chars line length, commas/spaces, etc.

[gilv]

@kxepal @rnewson I provided another patch, implementing most of the comments. Here is what is not implemented in this patch:

1. Remark on the AttFun. Not yet implemented. Original remark: “AttFun could be actually an attachment data or some fold function? Something went wrong if you have to do this.”
2. usage of couch_util:to_hex
3. still no unitests

[gilv]

@kxepal Can you please review the recent code?