Merge pull request #2 from md-systems/8.x-1.x

Update for interface changes.
Update some server property names.
Update the code for recent search_api changes.
Add config schema for the solr backend. Fixes #3.
Remove unneeded hook_view_api() implementation. Fixes #6.
Update hooks documentation. Fixes #4.
Restore the hook_cron() implementation.
Restore the 'Solr config files form' functionality. Fixes #5.
Restore the functionality of using a specific HTTP method.
Restore Basic HTTP authentication functionality.
Small hook_help() update for core change.
Update for FormStateInterface