FIX: Dry Client#deleteAll().
NEW: Implement and test realtime get feature, Client#get(), close #88

The test written is suggested in https://wiki.apache.org/solr/RealTimeGet
NEW: Rename Client#get by Client#realTimeGet, use Client#get to do arbitrary HTTP GET request.
FIX: Dry Client#spell, Client#search, Client#ping, Client#realTimeGet.