Ignfrance support travis

This PR replaces #94
See <https://travis-ci.org/ThomasG77/geopy> to check with my own IGN France key that everything work.

You will see below the comment from previous PR reported here

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
Having `sel_pl_attr` calls in this loop is convenient but unnecessarily expensive on strings. Could they move out to class or module level constants?