Use v8::Isolate as part of the key to the template cache

This fixes #34.

The v8 function templates used to pass objects from PHP to JavaScript world must not be shared between isolates.  Doing so anyways leads to segmentation faults as shown with multi-objects.phpt test submitted with issue #31

This patch adds the isolate pointer as another part of the cache key to omit reuse accross isolate bounds.

cheers
  stesie