Refactor so that requirements.txt matches setup.py install

`pip install -r requirements.txt` will have now have the same result like
installing with `python setup.py install`. Versions of packages are now also
more pinned to a specific minimum version and only restricted by the next major
version which should be tested before allowing usage in PyBossa.
This packages are for historical reasons even restricted
to their minor version currently:
* Flask-Login==0.2.3
* Flask-OAuth==0.12
* Flask-WTF==0.6
* Flask==0.10.1
* SQLAlchemy==0.7.8