Heroku deployment profile

Hi,

I did some modifications to make the project compatible with Heroku:
* Specify dependencies in `requirements.txt` and Heroku-specific ones in `req-heroku.txt`
* Add Heroku boilerplate (`Procfile`, `run_heroku`)
* Define a *gunicorn*-based config file called `heroku.ini`
* Remove *PasteScript* dependency in `setup.py` (conflict)

Hope it helps!

### Usage:
* Subscribe on [Heroku](https://www.heroku.com/) (free)
* Download and install [Heroku Toolbelt](https://toolbelt.heroku.com/)
* And do this:
```bash
git clone git@github.com:jdesboeufs/openfisca-web-api.git
cd openfisca-web-api
heroku create --region eu --buildpack https://github.com/jdesboeufs/heroku-buildpack-openfisca
git push heroku master
heroku open
```
Note: *Pandas* compilation = 3 minutes

If you want to use your own fork of **openfisca-france**, let's update `requirements.txt`!