Removing the requirement that Rails::Application be a singleton, and
reworking the way the main app is initialized.
Using instance eval instead of class_eval inside of Rails::Application
which changes the structure of configuring and creating a new
application. Creating instance methods linking up to class methods in order to handle this.
Revert "Using instance eval instead of class_eval inside of Rails::Application"

This reverts commit b39f872ef390b1afb185b6059892a5f24022c29b.