Add 'numpy', 'scykit-learn' and 'scipy' to requirements.txt

These are necessary to run `python demonstrate_scorer.py`.

Before installing these packages, make sure you have these installed:
* python-sklearn
* python3.4-dev
* libatlas-base-dev
* gfortran
Merge pull request #13 from he7d3r/master

Add 'numpy', 'scykit-learn' and 'scipy' to requirements.txt