Implement additional score calculators

[sashafrey]

• PhiSparsity - sparsity of phi matrix
• ThetaSparsity - sparsity of theta matrix
• ItemsProcessed - total number of processed items that contributed to the model
• TopWordsPerTopic - a matrix that shows topN words per each topic
• RandomThetaSnippet - a matrix that reveal a random snippet of Theta matrix (random N items)

Some of scores will require some improvements in scores framework:

• implement option to calculate statistics based on phi matrix (not on per-item basis)
• integrate with dictionaries

[sashafrey]

Done in https://github.com/sashafrey/topicmod/tree/marina_scores, integrated into master.