Local storage of exchange rates

I added the possibility to store the exchange rates locally (for performance issues) and to easily update them from command line (added a cron job example in the docs).
Also added a service for using the local data to perform currency exchanges.

If you think it's useful I would like to share this code with the rest of the world.
The code may not be perfect as this is my first PR but feel free to add comments and I'll try to improve it.