don't os.chdir to app.py directory

Per commit msg:

this was breaking local/development runs of the form:

`    $ python webdiff/app.py a b`

I couldn't see any reason why it was present / used.
I believe the comment on line 34 is still relevant. You can test that by building and installing your own source distribution with this incantation:

```bash
mkdir /tmp/webdiff-test
python setup.py sdist && cp dist/webdiff-0.6.2.tar.gz /tmp/webdiff-test
cd /tmp/webdiff-test
pip unintsall webdiff
pip install webdiff-0.6.2.tar.gz
```

I'm sure there's a tighter way to iterate on the distribution version, I just haven't found it.