src: fix windows build

Include node.h so the BUILDING_NODE_EXTENSION macro can work its magic
and exports V8 functions and types properly.  Fixes the following
build error:

    node.lib(node.exe) : error LNK2005: "public: __cdecl
    v8::Handle<class v8::Object>::Handle<class v8::Object>(class
    v8::Object *)" (??0?$Handle@VObject@v8@@@v8@@QEAA@PEAVObject@1@@Z)
    already defined in platform-win32.obj
    node.lib(node.exe) : error LNK2005: "public: __cdecl
    v8::Handle<class v8::FunctionTemplate>::Handle<class
    v8::FunctionTemplate>(void)"
    (??0?$Handle@VFunctionTemplate@v8@@@v8@@QEAA@XZ) already defined
    in platform-win32.obj

Fixes #30.