Support for Visual studio 2014

Adds builds against visual studio 2014 on appveyor (uses custom built node.exe from s3://mapbox/node-cpp11)

Before merging:

  - [x] tag node-pre-gyp@0.6.0
  - [x] find a way to grab the output of dumpbin and assert that it contains the right dlls when building against visual studo 2014