fix segfault in GlobalPropertyGetter() (closes #89)

GlobalPropertyGetter() is a static function. It can be invoked on
a context which has already been disposed off.

It turned out that there were two possible segfault reasons.

The most obviuos one was NULL pointer dereferencing.

However, there is no way to check whether the context is alive by
examining content of it or its sandbox. Both could look valid, but cause
segfaults in node. As a woraround, we add a self-reference which we clear
on destruction. A valid self-reference is a solid indication of a
valid context now.
> Thus, the weak callback won't get invoked while there is a reference to the wrapped object, because the garbage collector will not touch persistent handles.

In other words, you say that it's impossible to have a dangling pointer to context. But that's not the case. You can run `zombie` test suite with `browser.destroy()` uncommented in `test/event_source_test.js` to verify. It will segfault in approx. 3/4 of cases.