Pretty much rewrote the french translation

It was terrible and full of grammar mistakes. It was probably adapted
from a translation service.
Designated martini.RunOnAddr() to change host/port

Promoted martini.RunOnAddr as the method to use to run martini on a
different host/port.
Merge pull request #324 from mohamedattahri/master

New french translation, and documented RunOnAddr as the right method to change port/host