Added resolve nuget.exe from path

Added option to try resolve nuget.exe from `path` if not found
Added storing nuget.exe path to environment variable `NUGET_EXE`

This requires cake supporting this as proposed in [Cake PR #141](https://github.com/cake-build/cake/pull/141) or that build.cake is adjusted like [this](https://github.com/WCOMAB/PiFaceServer/commit/b0f27e673c6acab1e11d1f71b5a8719bc71f964e#diff-4d3634747586b964df9f7712bc3ec669R54)

I.e. AppVeyor log will look something like this
```
Build started
2  git clone -q --branch=master git://github.com/WCOMAB/PiFaceServer.git C:\projects\pifaceserver
3  git checkout -qf b0f27e673c6acab1e11d1f71b5a8719bc71f964e
4  .\build.ps1
5  Trying to find nuget.exe in path
6  Found C:\Tools\NuGet\nuget.exe 
7  Installing 'Cake 0.1.24'.
8  Successfully installed 'Cake 0.1.24'.
```