Add IGN France geocoder support

I've added the French IGN (National Geographic Institute) geocoder. It's an XML based geocoder.
I only use the native Python libs to not add new unwanted dependencies to the project.
It's tested with `make test` and `make pylint`, works on both Python 2 and 3.

To create a key for testing infrastructure, you can create API key for developers at <http://api.ign.fr/moncompte/login>.
In the left part, you will have to register with *S'inscrire à IGN API*. 
The authentication can be done with API key with referer or with API key with a couple username/password.
For test, I use the second solution. You will have to create a key **SIG** and tick various box about OpenLS (an XML based standard for geocoding) to create a key that make tests running.
Thanks for your feedback about my code and about the API subscribe process.

Cheers