Add default images for initial app header

As for know, c2cgeoportal scaffolds create templates that use non existing images (banner_left.png, banner_right.png, favicon.ico) located in a non existing directory (images/). When generating a new project, one then obtains an interface with broken images. This PR provides default images (based upon Camptocamp/Mapfish logos).

Here is an overview of the result:
![default_header](https://f.cloud.github.com/assets/1192331/1208356/129a6e2e-25cf-11e3-8f0a-09710695b357.png)