PoC: Add support for indexing the content of attachments using sunspot_cell

[znerol]

This PR adds support for text extraction of binary attachments using tika. Regrettably there is currently no centrally maintained version of sunspot_cell. The waterfield fork seems to be the most advanced one.

It is currently necessary to use xml as the sunspot update_format. The json format can be fixed with waterfield/sunspot_cell#1

The following config/sunspot.yml is working for me:

---
development:
  solr:
    hostname: localhost
    port: 8983
    log_level: INFO
    path: /solr/sunspot
    solr_home: solr
    update_format: xml

I'm using docker solr 7 for testing purposes with the core config from the sunspot examples with libs, fields, fieldTypes and ExtractingRequestHandler added (see attached sunspot-configset_cell.patch.txt).

This PR is in a proof-of-concept stage. The necessary config changes could be incorporated into the included solr config example. Also it would be nice if the search result would display a result snipped and not only the attachment...

[ncreuschling]

@KieranHayes This looks interesting.