Update documentation for W2k12 R2

[nhebling]

Hi,

the documentation refers to the node-gyp prerequisites and that Windows 7 SDK is required. Following these instructions is not possible on Windows Server 2012 R2 as the Windows 7 SDK was replaced by the Windows 8.1 SDK. Additionally it was not possible to install "lwip" with new NodeJS LTS release 4.2.1 as it caused build errors. A working setup in my case is the following, maybe it can be added to the documentation to give a hint what's required.

• Node.js v0.12.7 (LTS 4.2.1 did not work, anything in between was not tested)
• VS 2013 for Windows Desktop
• Opened VS 2013 developer command prompt
• Install node-gyp: npm install -g node-gyp --msvs_version=2013
• Install lwip: npm install lwip --msvs_version=2013