Moved WriteLiteral method to UmbracoViewPage

When we use a custom model through a custom controller, in our view, we
inherit from UmbracoViewPage, so the WriteLiteral method, which provides
preview badge and miniprofiler code, should be placed there.
Merge pull request #79 from sniffdk/6.1.4

Moved WriteLiteral method to UmbracoViewPage