Add the datagrid example

[jasongrout]

We should add https://github.com/phosphorjs/phosphor/tree/master/examples/example-datagrid to the repo so we have a standalone version of http://phosphorjs.github.io/examples/datagrid/index.html available.

[jasongrout]

CC @jzf2101

[jzf2101]

@jasongrout do we still want to move this over? I had thought you didn't want to reference the example in the docs?

[jasongrout]

I think we should have it in the demo. I think we should not have it in the docs. So yes, I think we still want to move it over.

[jzf2101]

So do you want us to subversion the repo and then build the website to show locally?