Simplified tree building

Tree building is now made outside and every existing nodes are in DOM
Added an emptyView and a droppableMixin
Bug fix nodeContent property binding
2 bug fixes

We need to observe node.@each to rebuild the tree and nodes are now an alias of treeContent
Multi selection handling with CTRL and SHIFT

Multiple rows can now be selected pressing CTRL key or SHIFT key
Merge pull request #1 from bobey/dev

Multi selection, drop on empty tree, bug fixes and code cleaning