Adds a schedule_json view which provides a /schedule/conference.json endpoint.

Of the kind that Carl uses for producing conference videos.

Also useful to feed into mobile schedule apps!

It is expected that someone might have to customize this for their own installation (PyCon Canada definitely did, with modifications to some of the models).

I have not _tested_ this, and it's slightly modified from what PyCon Canada did (we had additional "should we include this slot?" logic): https://github.com/pyconca/2013-web/blob/pyconca2013/symposion/schedule/views.py#L168

I know that a Proper API is planned, but until that's done, this is better than scraping `<table>`s. :)

Thanks again for Symposion!