Merge tag '3.1.0' into develop

Add nuget targets for .NET 4.5
Fix compile error in test project
Add PORTABLE45 to release configuration
Add appveyor.yml config file
Fix appveyor.yml
Remove obsolete nuget T4MultiFile

It's from back when we were using T4 templates
Visual Studio cleanup of project files and nugets
Set assembly versions back to 1.0.0

These are supposed to be changed as part of build process on build
server.
Reorganize build process

* Use Artifacts folder as build output, ignored from repo
* Use msbuild files to define paths to src and test projects
* Add batch files for building, testing, nuget and zipping
* Add NUnit and 7Zip tools to be able to build and test locally
Add version.txt
Add UpdateBuildDate.ps1

Needed for TeamCity standard build configuration
Delete duplicate version.txt
Allow running build scripts from different working directory
Restore nugets as part of build
Remove obsolete nuget T4MultiFile
Do not pause in build script

Appveyor does not support this
Change test project to .NET 4.5.1

Appveyor had issues with "implicit reference to .NET 4.0" in the .NET
3.5 version of Units.NET. Might be related to the OS and .NET
framework installed on VM.
Update paths for new test project framework
Update build badge to new build server
Remove appveyor.yml