Merge pull request #32 from sashafrey/master

Allow to restrict the set of topics in GetThetaMatrixArgs
Merge pull request #33 from sashafrey/master

Include token probabilities in TopTokensScore
Merge pull request #34 from sashafrey/master

Minor performance tweaks
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 tweak in topic_model.cc
cpplint fixes