Definition of the method getTree in the theme controller

	This method returns the HTML code needed to display the family tree of a theme
The attribute $tree has been added in the definition of the Thematique entity
Buggy: first try of automatic update of the tree attribute of a theme when adding it
Treeview works fine on a theme page but not on the roots page because the urls are not defined properly
Problem with the urls and the treeview fixed

	Definition of a controller in TreeController.php called by a template when the treeview is needed
	No $tree attribute in the Theme entity anymore
	ThemeHandler has come back to its initial state
Update of README