Haystack (search) integration

WIP

* Index events (including speaker's names, tags, department and location)
* Query events (including full-text search, faceting, suggestions)
* Provide schema for Solr

TODO:
* sort out boosting of results (prefer results in the near future)
* sort out date range facet
* index events on save/update/delete
* define what kind of results we want back: (only events or speakers as well for example)
* define what search we need to expose through the API
* define suggester

Type of the tag, e.g. BBC is an Organisation, Berlin is a Place...