Doc install

add an install script that:

- build navitia
- import data (by default it download a Paris dataset)
- run a front end and a kraken core

it is made more as an installation guide, but can be used as an out of the box build script

Note: I don't understand why the kraken is not killed at the end of the run.
Anybody has an idea ?
I do agree, but then how can you manage the postgres stuff ?