SENTRY_IGNORE_EXCEPTIONS now support wildcard '*' in Django

I was trying to use `django.request` logger to send uncaught Exception to Sentry via Raven. The problem is that I always get duplicated record logged by the `root` logger (presented on Sentry UI). After spending a few hours, I figured out that the `root` logger entry was created by `sentry_exception_handler`.

This PR add support to ignore all exceptions by specifying the following config in `settings.py`.

    SENTRY_IGNORE_EXCEPTIONS = ('*', )