Tile dates

This PR adds a new object and two new translations strings to the main page:
* `Crdppf.tileNames` : `Crdppf.tileNames['plan_ville_name']` and `Crdppf.tileNames['plan_cadastral_name']` contain the daily based name of these two layers
* OpenLayers.Lang.fr contains the two translation strings.

To activate the use of this option, add a `tile_date.yaml` file to the project's root folder. If this file is missing, then the project will use the default `plan_cadastral_c2c` and `plan_ville_c2c` layers.

Please take note of this option.