Complain loudly if missing request template context processor

Hi, this pull request alters behavior of the template tags if django.core.context_processors.request isn't in your settings.TEMPLATE_CONTEXT_PROCESSORS. Currently, the affected template tags fail silently - they output an empty string. This commit changes that so they raise an ImproperlyConfigured exception.

I was bitten by this when getting started with django_tables2, I've since fixed up my settings, but I figure this might save someone else a bit of debugging. Cheers!