SVG waterfall chart mapper

The previous waterfall mapper was just transforming to a JSON format that worked smoothly with d3. However that was no good from a progressive enhancement point-of-view, so I have changed it to dump out an entire HTML document, including the SVG chart and a table of raw data.

It uses handlebars to template the waterfall document and deliberately avoids splitting out styles and script to external modules so that the mapped data remains coherent regardless of the forwarder that is applied to it.

There is also a further change to mappers and forwarders, allowing mappers to specify a data type which modifies the forwarder behaviour. For the HTTP forwarder it sets the `Content-Type` header and for the file forwarder it sets the extension.