supports OAuth with GitHub

Actually not an authentication but should work fine for a time being.
Most of the code is copied from here:
https://github.com/cenkalti/github-flask/blob/master/example.py

At the same time SQLAlchemy is introduced.  Those DB thing should be
refactored out to other place some time.
cosmetic
add appropriate scope to auth request
handling multiple books
keep UI in English for the time being
register a book and its repository will be pulled
save a text and it will be pushed to github