Allow templates to be compiled when they contain a tilde (~).

I have templates with a tilde in them that get resolved later, however, the Razor engine doesn't compile with them. I'd like for them to be handled gracefully if they are not yet supported.

I created a naive ResolveUrl method and hooked it up to the RazorEngineHost.GeneratedClassContext.

I've left the larger problem of actually resolving urls, possibly using a VirtualPathProvider, as a separate issue.