replaced all references to numpy.linalg.svd with scipy's version && replaces numpy.linalgLinAlgError with scipy.linalg.LinAlgError

Hello Radim,

as discussed on the mailing-list and referenced in [ISSUE #158](https://github.com/piskvorky/gensim/issues/158) - here my very first ever pull request. 

Hope it is OK and will fulfill requirements. 

Greetings Sven