Save node's context as global persistent handle.
Save node's context as global persistent handle.

Part of fixes for rogerwang/node-webkit#33

littlstar/chromium.src,patrickm/chromium.src,patrickm/chromium.src,patrickm/chromium.src,ondra-novak/chromium.src,patrickm/chromium.src,patrickm/chromium.src,littlstar/chromium.src,patrickm/chromium.src,littlstar/chromium.src,patrickm/chromium.src,ondra-novak/chromium.src,littlstar/chromium.src,littlstar/chromium.src,ondra-novak/chromium.src,ondra-novak/chromium.src,littlstar/chromium.src,ondra-novak/chromium.src,littlstar/chromium.src,ondra-novak/chromium.src,patrickm/chromium.src,ondra-novak/chromium.src,ondra-novak/chromium.src,ondra-novak/chromium.src,littlstar/chromium.src,patrickm/chromium.src