change `path.exists*` to `fs.exists*`

The `path.exists*` methods have been moved to the `fs` module. c.f. [API changes between v0.6 and v0.8](https://github.com/joyent/node/wiki/API-changes-between-v0.6-and-v0.8)

Here are the following lines that need to be changed:

```
./petrify/lib/petrify.js:158: path.exists(dirname, function(exists){
./petrify/lib/petrify.js:282: path.exists(opt.output_dir, function(exists){
./petrify/test/test-petrify.js:11: path.exists(dirpath, function(exists){
```

Start with the three locations listed in petrify/lib/petrify.js and petrify/test/test-petrify.js, and review the linked Node.js API changes. Check the existing test flow, update each listed existence check consistently, then run the petrify test suite; done means no path.exists* calls remain in these locations and the tests pass.

Written by the indexing model from the issue text.