Use SetAlignedPointerInInternalField on node>=0.9.6

This fixes segfaults when using contextify with node>=0.9.6

node::ObjectWrap::Wrap and Unwrap in node>=0.9.6 uses
Set/GetAlignedPointerInInternalField instead of
Set/GetPointerInInternalField.
Merge pull request #59 from pyokagan/master

Fix segfaults with node>=0.9.6 due to changes in node ObjectWrap