contextify: ignore getters during initialization

The `context_` is not initialized until the `CreateV8Context` will
return. Make sure that it will be empty (by moving away initialization
from constructor) at start, and ignore getter callbacks until it will
have some value.

PR-URL: https://github.com/nodejs/io.js/pull/2091


Reviewed-By: Trevor Norris <trev.norris@gmail.com>