Support RenderPart

[tpetricek]

Here is one more change that I did in my fork: tpetricek/TomaspNet.Website@696a345#diff-4f91242bbeae924e249858f54ae18a15R11 (In case this does not scroll down automatically, go to the end of the page and have a look at the last 2 files).

The RenderPart method in Template.fs is called when your Razor code contains:

 @RenderPart("part.cshtml", new Model(...))

This is pretty useful feature. It was previously broken, because it was ignoring the layout root set when calling the Razor engine. I do not quite know how to best pass the layout roots to the template (so my build just uses ugly global variable...)