Create the v8::Context outside of CreateEnvironment.

This allows callers to supply which context is used in the environment.

@indutny

I wrote the original "node embedding api", if you want to call it that, which is used to embed Node into Plask.  Basically there are a few parts of the startup process that Plask needs to be involved in, so there is where Start(), Init(), etc came from.  There have been a few changes in Node that broke some of the API, so I have a few patches coming here to bring Node back to an embeddable state.

Thanks