Fixing issues with atomic saves 

This fixes #7.

Switching to [chokidar](https://github.com/paulmillr/chokidar) for better `fs.watch` support.  Chokidar fixes the issues that the built-in watch has with regards to atomic saves.

I removed the `require('path')` statement in `lib/watcher.js` as well since it was unused.

Also, the `marked` test for dox was failing since marked no longer includes the `readme` data that is required by the test (fresh `node_modules` install as of tonight), so I looked in the `node_modules` directory and saw that `xtend` provides the correct data, so made the switch to that so that the test cases would succeed.