milkman now depends on PIL

As of commit 01bdee23636b297064661988dc14ab010ac0a979, 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.