Add DateTimeFieldRenderer to show human-friendly timestamps.

This leverages the `humanize` package to do so.  Unfortunately that doesn't
seem to handle tz-aware times though, so we may need to revisit that at
some point...?

setup.py

@@ -75,6 +75,7 @@ requires = [
     # deprecated 'paster create' (and friends).
     'pyramid>=1.3a1',                   # 1.3b2                 1.4.5
 
+    'humanize',                         # 0.5.1
     'Mako',                             # 0.6.2
     'pyramid_beaker>=0.6',              #                       0.6.1
     'pyramid_debugtoolbar',             # 1.0