Add new feature for diversity search in Solr.

We proprose new feature for integrating diversity search in Solr. This feature enables users to select documents with keeping diversity based on specified fields. In addition, the feature supports selecting and merging documents according to given topic distribution. Please see attached pdf for more details of the usage.

Note that, this patch is trial codes and it is necessary to refactor them by the correct way. We'd like to know opinions about this feature, so please feel free to comment on this issue.