add a class method "tree_view" to show records in a tree view

show records in a tree view, sometime this can be very helpful in rails console.

Example:
`Category.tree_view(:name)`
will output:
```
root
 |_ child1
 |    |_ subchild1
 |    |_ subchild2
 |_ child2
      |_ subchild3
      |_ subchild4
```