milkman now depends on PIL

[Wilfred]

As of commit 01bdee2, importing
milkman throws ImportError if PIL isn't installed:

In [1]: from milkman.dairy import milkman
---------------------------------------------------------------------------
ImportError                               Traceback (most recent call last)
/home/wilfred/.envs/drawbridge/lib/python2.7/site-packages/django/core/management/commands/shell.pyc in <module>()
----> 1 from milkman.dairy import milkman

/home/wilfred/src/milkman/milkman/dairy.py in <module>()
      2 from django.db.models.fields.related import RelatedField
      3 
----> 4 from milkman import generators
      5 
      6 

/home/wilfred/src/milkman/milkman/generators.py in <module>()
      9 from django.core.files.storage import DefaultStorage
     10 
---> 11 from PIL import Image, ImageDraw
     12 
     13 

ImportError: No module named PIL

This commit adds PIL as an installation dependency in setup.py.