Issue with using objgraph in jupyter console

If I use objgraph in an IPython session things work as expected:

```python
In [1]: import objgraph

In [2]: a = 1

In [3]: objgraph.show_backrefs(objgraph.by_type('int'))
Graph written to /var/folders/zy/t1l3sx310d3d6p0kyxqzlrnr0000gr/T/objgraph-5iuhcjwd.dot (0 nodes)
Graph viewer (xdot) not found, generating a png instead
Image generated as /var/folders/zy/t1l3sx310d3d6p0kyxqzlrnr0000gr/T/objgraph-5iuhcjwd.png
```

However if I use ``jupyter qtconsole`` there is no output from ``show_backrefs``, and if I try and specify an output filename, nothing happens:

```python
In [1]: import objgraph

In [2]: a = 1

In [3]: objgraph.show_backrefs(objgraph.by_type('int'))

```

what could be going on?

Reproduce the difference between an IPython session and jupyter qtconsole using objgraph.show_backrefs(objgraph.by_type('int')), including an output filename. Compare whether graph output is generated or displayed in each environment; done means the qtconsole behavior is explained and a focused fix or documented limitation is verified.

Written by the indexing model from the issue text.