Add document loader

```
// Public: DocumentLoader takes a path and an object with update and remove
// functions and first calls update for every file in the path and then watches
// the path for file changes and calls update or remove as needed.
//
// path       - String path to directory to load and watch
// extensions - Array of extensions (each is a String)
// callbacks  - Object with update and remove functions
//
// Example Usage:
//
//   > var DocumentLoader = require("./browser/DocumentLoader");
//   > var path = "/tmp";
//
//   > var callbacks = {
//       update: function(details) {
//         console.log("Updated", details.filename);
//       },
//       remove: function(filename) {
//         console.log("Removed", filename);
//       }
//     }
//
//   > new DocumentLoader(path, [".md"], callbacks);
//   { path: "/tmp", extensions: [".md"] }
//
//   > var fs = require("fs")
//   > fs.writeFileSync(path + "/foo.md", "hello world");
//   Updated foo.md
//   > fs.unlinkSync(path + "/foo.md")
//   Removed foo.md
//
// Returns an Object.
```