option for disable dragging(issue#27)

[hangy233]

added a draggable option, give a false to disable dragging.

<Tree
  paddingLeft={20}
  tree={this.state.tree}
  onChange={this.handleChange}
  isNodeCollapsed={this.isNodeCollapsed}
  renderNode={this.renderNode}
  draggable={false}
  />

default value is true.

[DevAlien]

Could you try to solve the conflicts?

[hangy233]

solved the conflicts, you guys can merge if this is still needed.