Match exactly one case condition in a switch statement, and make sure 'default' only matches if no other case does (as a consequence).

Also, a minor issue was fixed where the 'default' symbol value would spill outside of the element scope.

This fixes issue #72 (see also http://stackoverflow.com/questions/8230502/chameleon-templating-default-value-in-talswitch/8253213#8253213).