Add prototype of computing vector representations of paragraphs.

This is not computation of paragraph embeddings as such: this is merely
computing the vector representation of the paragraph based on its words,
but taking into account their bias (related to their frequency in the
corpus --- so that a paragraph will be closer to its rarer words).
Merge pull request #2 from maciejkula/context_paragraph_vectors

Add prototype of computing vector representations of paragraphs.