Add NanScope to async_cb_handler

[rushidesai]

This fixes a crash on iojs v2.4:

Program received signal SIGABRT, Aborted.
0x0000003036c348d7 in raise () from /lib64/libc.so.6
Missing separate debuginfos, use: debuginfo-install bzip2-libs-1.0.6-14.fc21.x86_64 elfutils-libelf-0.161-6.fc21.x86_64 elfutils-libs-0.161-6.fc21.x86_64 libgcc-4.9.2-6.fc21.x86_64 libstdc++-4.9.2-6.fc21.x86_64 systemd-libs-216-25.fc21.x86_64 xz-libs-5.1.2-14alpha.fc21.x86_64 zlib-1.2.8-7.fc21.x86_64
(gdb) bt
#0 0x0000003036c348d7 in raise () from /lib64/libc.so.6
#1 0x0000003036c3653a in abort () from /lib64/libc.so.6
#2 0x0000000000cd7053 in node::OnFatalError(char const_, char const_) ()
#3 0x000000000082b685 in v8::Utils::ReportApiFailure(char const_, char const_) ()
#4 0x00000000009b347f in v8::internal::HandleScope::Extend(v8::internal::Isolate*) ()
#5 0x0000000000994438 in v8::internal::Factory::NewRawOneByteString(int, v8::internal::PretenureFlag) ()
#6 0x00000000009947b4 in v8::internal::Factory::NewStringFromOneByte(v8::internal::Vector, v8::internal::PretenureFlag) ()
#7 0x0000000000994d20 in v8::internal::Factory::NewStringFromUtf8(v8::internal::Vector, v8::internal::PretenureFlag) ()
#8 0x000000000083f2da in v8::String::NewFromUtf8(v8::Isolate_, char const_, v8::String::NewStringType, int) ()
#9 0x00002aaaab4d6078 in OZW::handleNotification(OZW::NotifInfo*) ()

from /tmp/t/node_modules/openzwave-shared/build/Release/openzwave_shared.node
#10 0x00002aaaab4d6d3f in OZW::async_cb_handler(uv_async_s*) ()

from /tmp/t/node_modules/openzwave-shared/build/Release/openzwave_shared.node
#11 0x0000000000d54eb6 in uv__async_event (

loop=0x133a800 <default_loop_struct>, w=<optimized out>,
nevents=<optimized out>) at ../deps/uv/src/unix/async.c:92

#12 0x0000000000d54f73 in uv__async_io (

loop=0x133a800 <default_loop_struct>,
w=0x133a9c8 <default_loop_struct+456>, events=<optimized out>)
at ../deps/uv/src/unix/async.c:132

#13 0x0000000000d64325 in uv__io_poll (

loop=loop@entry=0x133a800 <default_loop_struct>, timeout=-1)
at ../deps/uv/src/unix/linux-core.c:345

#14 0x0000000000d55a46 in uv_run (

loop=0x133a800 <default_loop_struct>, mode=UV_RUN_ONCE)
at ../deps/uv/src/unix/core.c:328

#15 0x0000000000ce2040 in node::Start(int, char**) ()
#16 0x0000003036c1ffe0 in __libc_start_main () from /lib64/libc.so.6
#17 0x0000000000675a0d in _start ()