--directory option, allowing serving a different directory tree

I'm using [sphinx](http://sphinx-doc.org), which tends to structure things like:

```
mydoc/
   Makefile
   build/
      html/
   source/
      index.rst
      otherdir/
          index.rst
```

so I want to watch the source/ directory, run a command and serve up build/html/. Also, it makes more sense to keep the `Guardfile` next to the `Makefile`.

This simple patch allows me to run

```
    livereload -d build/html
```
with a very simple `Guardfile`

```
@functools.partial
def build_sphinx():
    import subprocess
    subprocess.call(['make', 'html'])
Task.add('source/', build_sphinx)
```