Add drag and drop functionnality from outside of the gantt chart.

Need some code cleaning and refactoring
Test
Revert "Test"

This reverts commit d29e4f5a947023dae2bdb5602967e2ca463bc27b.
Merge branch 'feature/drag-and-drop' into develop

Conflicts:
	assets/angular-gantt.js
	assets/angular-gantt.min.js
	assets/demo.js
	demo.html
	src/gantt.directive.js
	src/ui/task/task.directive.js
	template/gantt.tmpl.html
Merge branch 'feature/drag-and-drop' into develop