Stop searching config if we reach root directory

Fix #19.

Since project can be located not under `HOME`, we should stop searching config if we reach root.
There appears to be no good way to get root path in Windows, that's why assume that if current dir has no parent dir, we're in root.

![giphy](https://f.cloud.github.com/assets/872004/1817124/f84d9ace-6f74-11e3-8f9f-42918e8a0020.gif)