Feature extraction text ldalsi vectorizers

I would like to send a pull request on features I have added to the sklearn.feature_extraction.text module.

I have added two new classes, LdaVectorizer and LsiVectorizer, in the above module, to support extracting latent topics from text documents. As a result, a collection of text documents can be transformed into a set of topic weights in a much lower dimension.

To support the above added features, I also added one example, text_topics_extractor.py, in the examples directory. In addition, I created one section in doc/modules/feature_extraction.rst, to describe those two vectorizers in more detail.