rename stuff like options.js and /assets/ to names more sourcejs related

Now that install by dependency is going to become the official way and the only way in 0.7.x I think we should think about being a bit less invasive upon people's environment.

Imagine if every npm dependency just asked to add a options.js to the root of the folder? I propose we rename it to something like sourcejs.config.js if that's possible.

Also I think all template-overrides, assets overrides and core-overrides could be in a sourcejs folder by default, instead of in the root. Another options would be to rename them to sourcejs-assets, sourcejs-templates.. etc.