Add dependency graph widget (via Stratopan) GH #1168

This adds a "Dependency graph" link beneath the dependency list (but only if the release has dependencies). The graph itself appears in a modal that uses an iframe to load the content from Stratopan.  This required adding `boostrap-modal.js` to the JS kit.

These size of these graphs can vary greatly, so it was hard to come up with an interface that would accommodate most of them without being too obtrusive. There is a link to open the graph full-screen in a separate window, which is handy for large graphs.

Personally, I like the modal design because it doesn't take you away from the MetaCPAN site, and the iframe loads in the background so it (usually) feels very responsive. If you want to see something different, please let me know.

It looks right to me on latest Firefox, Chrome, and Safari. I haven't tested on older versions of those nor any version of IE or Opera.

On the Stratopan side, I have set up a separate application to serve this stuff (at http://widgets.stratopan.com). Currently there are 8 workers supporting it. If necessary, I can add more and/or rewrite it to be non-blocking.