Introduced tree.

[exabugs]

For simplify, introduced 'Tree' class.

I want to use 'react-dnd' for Drag&Drop.
( https://github.com/react-dnd/react-dnd )

Self recursive structure is not to work.

• Self recursive structure.

class TreeNode ...
  render() {
        ...
        <TreeNode>
  }

[coveralls]

Coverage increased (+0.07%) to 98.624% when pulling 5ff9371 on exabugs:feature/introduced_tree into 438c4f5 on alexcurtis:master.

[maximilianoforlenza]

@exabugs Hi! Can you update this PR?