Enable building with Visual Studio 2014

This makes two minor changes that will have no impact for normal usage.

But, if `--toolset=v140` is passed then:

 - Then the `msbuild_toolset` will be set. An empty string means that whatever compiler default node-gyp has will be used. Because we pass `--msvs_version=2013` that is the default compiler (aka MSBuild "12"). But for Visual Studio 2014 builds we need MSBuild "14" and `msbuild_toolset="v140"` triggers this. Once gyp [officially supports visual studio 2014](https://code.google.com/p/gyp/issues/detail?id=446) and node-gyp updates to it then the default could be chosen through the `msvs_version` flag. However until then the way to do this is `msbuild_toolset`.
 - Also node-pre-gyp 0.6.x will understand this option and published builds will go into a remote directory like `gdal/v0.3.0/v140/<binary>` directory. Note: if `--toolset` is not passed then the double `//` will be collapsed and the standard path will be used as normal: `gdal/v0.3.0/<binary>`.

After merging this we will need another quick tag. I'm happy to run this out.