Implement parameter parsing and better hacking in 'generator.word2vec.py'

Implemented the parsing of parameters in the 'generator.word2vec.py' module's 'create_embeddings' function, with a correct docstring. Updated the corresponding unittests. Implemented a 'data_format' = 'iterator_generator' in the Corpus class, so as to easily be able to use a real Corpus class defined from a list of list in the unittests. Corrected formatting here and there.

Resolves: #18