Replace deprecated calls to V8 API by newer equivalents

Hi again,

I've replaced calls to deprecated V8 API functions with newer equivalents.  This is mainly passing around some more v8::Isolate instances and adopting new callback function scheme.

This work is based on my earlier pull request #21.  I've split this into two pull requests to allow for quicker merging of the former, since it is far more critical as it already prevents this module from building.

cheers,
  stesie