Reduce size of gensim distrib

For the current moment, gensim distribution is very heavy (>60Mb for `.tar.gz`). Need to significantly reduce this value, for example, remove notebooks from tar.gz and so on.

**How to produce `tar.gz`**
`python setup.py sdist`

**How to control content in `tar.gz`**
Change [MANIFEST.in](https://github.com/RaRe-Technologies/gensim/blob/develop/MANIFEST.in)