Fixed up some unit tests with NiceUrlProvider to ensure that the second node under the root get's
the correct URL assigned.
Added unit test to ensure that route caches are persisted correctly.
Updated the references between context objects since a DocumentRequest should only be set when rendering
a front-end doc, not in the back office.
Fixes the LookupByNiceUrl to work with non directory paths.
Fixes the UmbracoContext.ClientUrl to work properly.
set at runtime. Exposes UmbracoContext via DocumentRequest which simplifies some things. Removes
dependency on UmbracoContext from DefaultRoutesCache, now InPreviewMode is checked before passing
processing to the IRoutesCache provider. Changed NiceUrls to NiceUrlResolver.
