add branch check

[hualuomoli]

add check icon before folder icon

[daerogami]

@hualuomoli This is a good feature; however, it appears that it is 'always on'. In order to merge this it will need to be disabled by default and enabled by a directive option.

Example:
<abn-tree tree-data = "my_treedata" tree-control = "my_tree" checkable></abn-tree>