Add NanScope to fix crash with node-webkit/x86_64

With node-webkit 0.11.5 running on Ubuntu 14.04.1 x86_64,
SerialportPoller::callCallback would exit with V8 error: Cannot create a
handle without a HandleScope. This occurred when constructing the
v8::Handle for argv[1]. Adding a NanScope() to the top of the function
resolves the issue.

This may be related to voodootikigod/node-serialport#379, but it's not
clear from the bug report.