Updated post to handle noncommited files.

post would rename a draft file to a post file, but it wouldn't handle
a draft that had never been commited before. Now it checks if the draft
you want to post has already been commited and and if not, doesn't rename
the nonexisting file.
Posting things-like-about-c++11.markdown
Adding C++11 draft.

This will be a post about the stuff I've used in C++11.