added tox configuration to run tests on django 1.4 and 1.5
models: stop using verify_exists, removed on 1.5
templates: fix old style {% url %}
files: File.mode attribute is behavior change

On 1.4 it was None by default:
 - https://github.com/django/django/blob/stable/1.4.x/django/core/files/base.py#L18

1.5 don't add the attribute:
 - https://github.com/django/django/blob/stable/1.5.x/django/core/files/base.py#L20
commands: handle behavior change on tests

From https://docs.djangoproject.com/en/1.5/releases/1.5/#minor-features:

"Management commands do not raise SystemExit any more when called by code
from call_command. Any exception raised by the command (mostly CommandError) is propagated."
tests settings: add SECRET_KEY, failed on 1.5
ignore test generated files
operations: fix a call to traceback.format_exc() and how it's tested
Support django 1.5