SVG file download

Added a `download` button to the examples to enable SVG fles download.

I didn't add the logic to the sequence-diagram lib as I my implementation depends on a [js lib](https://github.com/felipesabino/js-sequence-diagrams/blob/gh-pages/index.html#L188) from [phaistonian/SVGInnerHTML](https://github.com/phaistonian/SVGInnerHTML) to enable innerHTML for SVG tag elements and I wasn't sure if the lib itself should depend on it.

I can refactor it if you think this dependence is ok.