Added PNG support to ipyD3: better nbconvert compatibility

Following the suggestion of the original author of ipyD3, I added PNG capability to my fork of his ipyD3. Then, selecting "png" instead of "html" generates output that appears nearly identical. Example invocation:

d3.render(mode=('show','png'))

The disadvantage of PNG over the former HTML rendering is that it precludes any possibility of mouse interaction and animation. The advantage is that nbconvert will convert multiple PNG D3 renderings in the same Notebook, whereas with the HTML renderings nbconvert seemed to give up after the first one. This is using the last version of nbconvert before it was merged into the IPython project. I have not tried the beta versions of IPython 1.0; I'm waiting for the Anaconda release.