Support system python

Hi Roland,

i am using djangorecipe 1.3 with buildout 1.6.3. It works fine but when i tried to use the following buildout settings they don't work as expected.
```
include-site-packages = true
exec-sitecustomize = false
allowed-eggs-from-site-packages =
    lxml
    PIL
```
Turns out djangorecipe needs to be updated to support a system python and these settings. I  updated djangoreipe according to the buildout docs: http://pypi.python.org/pypi/zc.buildout/1.6.3#updating-recipes-to-support-a-system-python

Maybe these changes are useful for you and other peoples so feel free to merge them back into your project.

This is my first time contributing to a foreign project, so please be patient if i did something wrong ;)

Best regards,
Martin