add blacklight:check rake tasks for checking solr and controller configurations

[cbeer]

I'm not sure how widely desired this is, but I think it will be handy for trying to teach Blacklight, and hopefully useful in other contexts too. Here are the important checks I could think of:

$ rake blacklight:check:solr
[http://127.0.0.1:8983/solr/]
 - admin/ping: OK

$ rake blacklight:check:controller
[CatalogController]
 - find: OK
 - get_search_results: OK
 - get_solr_response_for_doc_id: OK

$ rake blacklight:check:controller VERBOSE=true
[CatalogController]
 - find: OK
    status: 0
    numFound: 30
    doc count: 10
    facet fields: 9
 - get_search_results: OK
    status: 0
    numFound: 30
    doc count: 10
    facet fields: 7
 - get_solr_response_for_doc_id: OK
    status: 0

$ rake blacklight:check:controller[BookmarksController]
[BookmarksController]
 - find: OK
 - get_search_results: OK
 - get_solr_response_for_doc_id: OK

Fixes #812