Feature/multiple tmpl langs support

It needs a little more testing and cleaning up, but would you be interested in this?

When I first tried Skinny (today), I didn't realise that SSP templates are the default and you have to override `scalateExtension`. I wasted a lot of time wondering why it couldn't find my Jade template.

This is an attempt to solve that problem by transparently supporting all Scalate template languages.
Now there is no need to specify the Scalate template extension, except in special cases.