Add tko-switch-case.js

[danieldickison]

This is a port of the original knockout-switch-case.js to work with the current version of tko. I initially tried to do a minimally invasive and backwards compatible change in knockout-switch-case.js itself but I couldn't get it to work well with the wrapped if binding handler now being implemented as a subclass BindingHandler while the switch/case handler keeps its init/update implementation.

I didn't port the case.visible binding as, from what I understand, dots in binding names are no longer supported.