New Uize.Test subclasses + Uize.Comm updates + other goodies

This pull request is highlighted by new `Uize.Test` subclasses aimed at providing helper methods for testing `Uize.Class` subclasses, `Uize.Widget` subclasses, and widget bindings.

There are also improvements to `Uize.Comm` and `Uize.Comm.Ajax` for error handling. The AJAX object would die before if an error occurred.

There are other miscellaneous fixes outlined in the commit logs.
Is this approach ok for copying binary files?