Removing View#__toString().

As PHP can't throw an Exception in this method it makes working with
them difficult. Past iterations have attempted:

* Rendering the exception as a string. This leaks information about the
  host system.
* Rendering an empty string. This makes debugging errors in development
  extremely difficult.

Instead, call render() explicitly on the view, where any Exceptions will
be thrown and caught normally.
View now only checks the file exists on render().
Updating StringResponseListener to render a View.
Removing insert() and load() methods from View.

These methods revert control to the template. Inheritance isn't
supported so there should be no illusions it is!
Major cleanup of View.
Moving ViewNotFoundException into the view namespace.
Adding ViewCreatorException.
Fixing a bug in View#addValues() and improving test coverage.
Merge branch 'view'