siblings nodes' height are not the same?

Hi msbanik,

The library is pretty cool, but I wonder can siblings nodes' height be the same?

For example, code:
```
from drawtree import draw_level_order
draw_level_order('{3,9,20,1,2,15,17}')
```
it generates:
```
      3
     / \
    /   \
   /     \
  9      20
 / \     / \
1   2   /   \
       15   17
```

Can node 1,2 and node 15,17 be in the same level?

Thank you.