Added default significant word into parser
Add to summary at least 1 sentence if possible
LSA: Added reduction of dimensionality parameter
Sentence is responsible for tokenizing to words

Sentence has new dependency - tokenizer. Tokenizer
returns only words that contains letters from now.
Before it was responsibility of sentence to clean
words.
README note - make sure Py is installed
Null stemmer moved to package 'nlp.stemmers'
Changed summarizer API - pass document on call

Previous implementation passes document to
constructor. So there was 1 summarizer per document.
I really don't know why I done it this way :/
LSA: don't check dependencies until summarizer is executed

This allows using CLI without NumPy & SciPy.