Add about zero-size vector

There is nothing about zero-size vector in N3337.  Implementation of the
libraries is investigated by @melpon.  Thanks!

https://twitter.com/melponn/status/413714218546786304
Merge pull request #87 from rhysd/about_zero-size_vector

vector のサイズが 0 のときの data() メンバ関数の挙動について