Updated the readme with links.
Added ant and pheromone types.

- Now their can exist several ant types (or species ?).
- Each ant type drops its own pheromone types.
- Food can have a type also and is consumed only by ants of the same
type.
- Changed the code to create a Pheromone class stored on the edges
instead of only one number.
- Updated to readme to talk about the more important point of the model
!! (that was absent of the description, ha ha).