Adds a "FlashContext"; Ensures that SessionController actions are mapped _per instance._

- FlashContext retrieves a "flash message" from session storage. This message is erased
from session storage once it has been read by the controller.

- SessionController's NewInstance() method now creates a _unique_ actionMap for
each request-facing instance.

(Since it is a ref type, context variables were nil when accessed from within an Action)

- Context Chain and router now work together to test if contexts can be applied to routes.
This is done once per route at runtime when the LoadRoutes() method is called.

(This only applies to routes using Context Chaining.)
Merge branch 'develop/refactor-router'