Add PIPELINE_ENABLED setting.

This was mentioned as the preferred setting name in issue #178, and I needed it, so here's the implementation; pretty simple of course. Adds support for a ``PIPELINE_ENABLED`` setting defaulting to ``not DEBUG`` (so it's backward compatible).

I fixed the existing tests, and would have added more but it seems there currently aren't any tests for the templatetags (or at least I couldn't find them). I suppose those tests could be added with this pull request, but that was more than I had time to set up at the moment.