Add requirements.txt to gh-pages branch

Replaces #100 , so as to allow merge into `gh-pages` branch: add requirements.txt with necessary dependencies to run validator and lesson build process. Tested on clean virtual env (python 2).
Please take `requirements.txt` out of this PR - once #102 is merged, we'll merge from `core` into `gh-pages`.  I agree the two-branch strategy is confusing - would you find two repos (`lesson-core` and `lesson-template`) easier to track?