add setup.py

This setup allows osmviz to be installed with pip, for example with this in requirements.txt:

```
-e git://github.com/olibook/osmviz.git#egg=osmviz
```

You can do:

```
pip install -r requirements.txt
```

And it'll install directly from Github. You can then import as normal:

```
try:
    from osmviz.manager import PILImageManager, OSMManager
except ImportError, e:
    sys.exit("ImportError: %s.\n"
              "OSM Viz module needed, available from\n"
              "http://cbick.github.com/osmviz/\n\n" % str(e))

```