impletement the LDA with the varitional inference and gibbs sampling.
###Vartional inference by python
The efficiency of Python language is unsatisfied. So next time may use C++.
###Gibbs sampling by python
follow the instruction https://people.cs.umass.edu/~wallach/courses/s11/cmpsci791ss/readings/griffiths02gibbs.pdf
note that the word-topic distribution is different from the origin LDA model.