Split some logic of TopicModel class into separate TokenCollection class
Introduce TopicModel::CalcNormalizers()
Do not maintain n_t_ on updates of topic model. Use CalcNormalizeres when n_t_ is really needed.
Minor tweaks in topic_model.[h,cc]
Fix crash in example04_online_algorith.py
Merge pull request #37 from sashafrey/master

Improve performance of TopicModel::ApplyDiff