Module

- new version of module.xml definition (module-2_1.xml) :
    - category of module
    - required version of Thelia : min, max 
    - documentation directory
    - supported languages 
- Possibility to install module from the back office
- new smarty plugin ```flash``` to use the flashMessage of the request
2.1.0-alpha1 is already released. 
Fill the 2.1.0-alpha2.sql file instead of this one.