Spress 2.0 site structure

[yosymfony]

The site structure for Spress 2.0 is so different respect to prior version. The new structure has the configuration data and the output folder at the first level. The main content is placed at src with a folder for includes, layouts, plugins and content. The new structure doesn't need a underscore prefix for folders with special meaning

site
|- build
|- src
| |- includes
| |- layouts
| |- plugins
| |- content
| | |- posts
| | |- index.html
| | |- ...
|- config.yml
|- composer.json

Another issues related with this: #23