BUG: Fixed py3 syntax error in gensim/models/lda_dispatcher.py
BUG: Fixed py3 syntax error in gensim/models/lsi_dispatcher.py
BUG: Fixed two py3 syntax errors in gensim/models/lsi_worker.py
Added two keyword arguments to `LdaModel().__init__()`

`gensim/models/ldamodel/LdaModel().__init__()` now has the keyword arguments
`iterations=50` and `gamma_threshold=0.001`, so the method behaves the same
as `LdaModel.init()` method of the py2 develop branch.
Changed two `LdaModel` attribute names

Changed `LdaModel.VAR_MAXITER` and `LdaModel.VAR_THRESH` to
`LdaModel.iterations` and `LdaModel.gamma_threshold`, as these are
the attribute names that are also used in the main py2 develop branch.