Better file structure for themes

I think we should reconsider the file structure for themes. Currently, it includes a `static` and a `templates` folder and both must be declared in `documentjs.json` for it to work.
### Suggestions
#### 1. Allow the `documentjs.json` to accept one parameter for the entire theme like:

```
"siteDefaults": {
"theme": "themes/my-awesome-theme"
},
```

This declaration would cause DocumentJS to get everything from this folder.
#### 2. Change the folder structure

The current folder structure with `static` doesn't make sense because `less` is not static and this folder also contains two mustache template files. Plus, it's messy having all the JS and other stuff in there.

I think it would be easier to manage if there were only 4 folders inside the theme folder:
- `templates` - all the mustache templates
- `css` or `less` - all the style files
- `images` - if any
- `js` - for the javascript

Any other files could also live in these folders and would be copied to the output directory.