removed duplicates from appearing in list when calling open post/draft multiple times.
added condition to check whether the drafts directory exists.
removed duplicates when promoting post to draft. Posts and Drafts list sorted by most recently modified.
updated sorting of list according to last modified.
Merge pull request #17 from fk128/master

Fixed bug with duplicate entries in Quick Panel

- Fixed duplicates from showing in Quick Panel
- Added directory check (in cases where drafts/posts paths do not exist)
- Added sorting of posts by last modified date