Have removed ResolverBase as we cannot rely on manually setting the current singleton object since applications outside of the standard umbraco web application
might be using these singletons and would expect they already be setup. Have changed all current resolvers to manage their
own singleton instances and sealed them.
and are just adding the known resolvers on app startup and commented out the other resolver stuff for now.
Fixed up xpath statement for resolving the first document, site now renders :)
already found. Have added some notes to some classes that need to be fixed up. Removed the PluginTypeResolver
from the ApplicationContext since it is a utility and isn't exposing application objects.
Updated ApplicationStartupHandler to use it and to not register on the static method, the real method is now
used and is called on app startup (much cleaner). Added tests for PluginTypeResolver. Added events to
UmbracoApplication and virtual methods for developers to override.
