respect stochastic SVD params in LsiModel

[piskvorky]

Currently, the extra_samples and power_iters parameters to LsiModel are only used in the multi-pass variant (=when onepass is False).

Pass them also to the one-pass variant, for consistency and greater flexibility.

Alternatively, mention explicitly in the docs that they are not used, to avoid confusion.

This issue was brought up by Brian Murphy at http://groups.google.com/group/gensim/browse_thread/thread/6de956d4300ab7da

[piskvorky]

Fix outline:

In file gensim/models/lsimodel.py, the internal calls to SVD are done from LsiModel objects via calls to Projection objects (found in the same file).

• wherever there is an internal call to SVD (i.e. lines such as update = Projection(self.num_terms, self.num_topics, job) in LsiModel, pass on self.extra_samples and self.power_iters to the Projection constructor as well.
• make Projection accept these extra constructor parameters, and use them for its internal stochastic_svd call, replacing the hardwired P2_EXTRA_ITERS and P2_EXTRA_DIMS values.
• add a unit-test that checks all works as expected