Implement app:draft mode into atom api

“RFC5023 The Atom Publishing Protocol” suggests to implement “app:control” element and “app:draft” element.
It makes entry to be configurable draft mode.
[http://www.ricoh.co.jp/src/rd/webtech/rfc5023_ja.html#sec-13-1](http://www.ricoh.co.jp/src/rd/webtech/rfc5023_ja.html#sec-13-1) (Japanese)
[http://tools.ietf.org/html/rfc5023#section-13.1](http://tools.ietf.org/html/rfc5023#section-13.1) (English)

So I implemented them into atom.vim.
Please review this