Index operations tweaks (Ensure, Drop, Rename).

  1) MongoClient::ensureIndex() now accepts name of index.
  2) Name of index is used on UI (instead of key name).
  3) MongoClient::deleteIndexFromCollection() simplified.
  4) MongoClient::renameIndexFromCollection() correctly
     performs rename of Index name.
  5) MongoClient::getIndexes() now gets Index names, not key names.
  6) Minor formatting tweaks.