Issue #188 Fix JSON Exporter for Symlink Nodes

[JaimeSandoval]

from anytree import Node, SymlinkNode
from anytree.exporter import DotExporter, JsonExporter, DictExporter
root = Node(name="root")
a = Node(name="a", parent=root)
b = Node(name="b", parent=root)
c = SymlinkNode(target=a, parent=b)
exporter = JsonExporter(indent=2, sort_keys=False)
print(exporter.export(root))

Expected Response: {'name': 'root', 'children': [{'name': 'a'}, {'name': 'b', 'children': [{'name': 'a'}]}]}
Given Response:

{
  "name": "root",
  "children": [
    {
      "name": "a"
    },
    {
      "name": "b",
      "children": [
        {
          "name": "a"
        }
      ]
    }
  ]
}

[coveralls]

Coverage decreased (-0.006%) to 99.754% when pulling 4fac651 on JaimeSandoval:master into d63289b on c0fec0de:master.