Cannot cleanup globals on a context

[huubbouma]

It's not possible to clean all globals on a DuktapeContext. This improvement would help reusing an existing context instead of recreating a new context every time (which seems to lead to a memory leak, but this is something for another issue)

[stefano]

The right approach here would be to throw away the old context and create a new one. In this case the memory leak prevents that. I will investigate what's causing the leak.