Tree Plugin

This implements Tree Plugin, based on #219 

Demo is available at http://rawgit.com/angular-gantt/angular-gantt/tree/demo/dist/index.html

- [x] Create tree plugin with [UI Tree](https://github.com/JimLiu/angular-ui-tree) dependency
- [x] Add hierarchy properties in model and support them in display
- [x] Support clicks on tree handles (collapse/expand)
- [x] Disable drag/drop
- [x] Support Odd/Even styles properly
- [x] Display style of rows on the whole width
- [x] Make configured sorting works on each hierarchy level
- [x] Add API method to refresh hierarchy based on model data
- [x] Tests