Add aver.exe as a tool to the project

aver.exe is a tool which patches AssemblyInfo.cs and .nuspec files with
semantic versions defined in version.txt files.
Add version.txt file

The file is read by aver.exe to determine how to patch versions of
assemblies and nuget-packages.
Add Appveyor settings .yml file

First try specifying the build config via the appveyor.yml file.

- Using aver.exe to patch versions
- Building only release configuration
- Restoring nuget packages before build
- Publishing nuget packages to project feed
- Tests should be discovered automatically
Merge remote-tracking branch 'origin/master'