added inference tooling to HDP as per implementation in LDA - uses LDA equivalent topics (have been using this method in run_online_hdp from CW with good success)
bug fixes from testing lda inference on pre-inferred data
handle empty docs properly - inference will have gamma all zeros, __getitem__ returns empty list
Merge pull request #153 from terramars/develop

added inference tooling to HDP as per implementation in LDA