Add Flask-Moment to requirements.txt

Hey.

I tried to deploy your project on my system.
I create virtual env, and install dependencies via:
```
pip install -r requirements.txt
```

Then i type python manage.py createall and get error:
```
python manage.py createall
Traceback (most recent call last):
  File "manage.py", line 4, in <module>
    from app import create_app, db
  File "/home/nless/Documents/projects/restful-todo/app/__init__.py", line 3, in <module>
    from flask.ext.moment import Moment
  File "/home/nless/.virtualenvs/vanilla-flask/lib/python2.7/site-packages/flask/exthook.py", line 87, in load_module
    raise ImportError('No module named %s' % fullname)
ImportError: No module named flask.ext.moment
```

So, fix in my patch.

My system:
* Fedora 20
* Python 2.7.5