Add Python-Twitter module to credits list.
#25: Optimise to only re-load results when changing highlighting if a
search has been run.
#23: Add Solr/WebApp availability checks to indexer to abort if either
not available.
- Add ping handler to WebApp.
#24: Handle Solr unavailability better in App.
- Avoid Terms thread looping continuously.
- Display error message instead of broken results page.