fix Pillow dependency

I think the whole "soft dependency" thing with PIL/Pillow should be the other way around. If you don't have any of them we should use the Pillow (de facto standard). If we already have PIL (as I see in Vagrant/setup.sh is installed in a seperate step) then we can use it. Also try to install this with setuptools first.

Without this fix you can't install TileStache right now. On a fresh virtualenv ``pip install Pillow TileStache==1.49.11`` gives:
```
Downloading/unpacking Pillow
  Downloading Pillow-2.6.1.tar.gz (7.3MB): 7.3MB downloaded
  Running setup.py (path:/tmp/test/build/Pillow/setup.py) egg_info for package Pillow

...
Downloading/unpacking TileStache==1.49.11
  Downloading TileStache-1.49.11.tar.gz (208kB): 208kB downloaded
  Running setup.py (path:/tmp/test/build/TileStache/setup.py) egg_info for package TileStache

Downloading/unpacking ModestMaps>=1.3.0 (from TileStache==1.49.11)
Downloading/unpacking simplejson (from TileStache==1.49.11)
Downloading/unpacking Werkzeug (from TileStache==1.49.11)
Downloading/unpacking PIL (from TileStache==1.49.11)
  Could not find any downloads that satisfy the requirement PIL (from TileStache==1.49.11)
  Some externally hosted files were ignored (use --allow-external PIL to allow).
Cleaning up...
No distributions at all found for PIL (from TileStache==1.49.11)
Storing debug log for failure in /home/slafs/.pip/pip.log
```