Change Atom parser to analyze draft status in "app:control"

Atom should recognize and parse "app:draft".
Please refer to the followings;
    http://tools.ietf.org/html/rfc5023#section-13.1
    http://www.ricoh.co.jp/src/rd/webtech/rfc5023_ja.html#sec-13-1
Merge pull request #14 from moznion/feature/atom_parse_draft

Change Atom parser to analyze the draft status in "app:control"