Support for the "use strict" directive in JS

As I tried to use the example-code in "mqttws31.js" and use the "use strict" directive ([see w3school for definition](http://www.w3schools.com/js/js_strict.asp)) I've got an error when debugging in Firefox:

> assignment to undeclared variable client

And the Message would not be transmitted. So I think, that the "use strict" directive isn't supported yet. It would be nice to support that in the Future if possible, because it was said that:

> - Strict mode makes it easier to write "secure" JavaScript.
> - Strict mode changes previously accepted "bad syntax" into real errors.