Update debug.py

500 error debug response is nerfed as of Django 1.4. The change 
resulted in 500 server errors only returning the traceback body, and 
not the nice interactive debug response.

This change makes working with AJAX in >1.4 very difficult compared with
<1.4. 

My change makes this nerfing optional. Just set NERF_DEBUG_ON_AJAX
to False in your settings.py