Most of the time when running our build scripts we want echo to be off to limit useless output. This commit allows us to keep it off by default, and turn it on when we need it by defining the "EchoOn" variable.

Adding a hack to workaround some new `VsDevCmd.bat` behavior related to it changing the current directory.

* Enable building and testing on machines with only Dev15 Preview 3 installed

1. Auto-detect the VS common tools path
2. Convert analyzer samples to import Roslyn targets
3. Ensure dependencies are copied to the output directory
4. Workaround: Use local Microsoft.Net.Compilers.props until we move to a compiler toolset with https://github.com/dotnet/roslyn/commit/05c12ebfcdd08a02dbceded5327a8da7a7df23be
5. Replace references to Microsoft.VisualStudio.ComponentModelHost.dll (VisualStudioReferenceAssemblyVersion) with a nuget reference to RoslynDependencies.Microsoft.VisualStudio.ComponentModelHost
6. Add nuget references to Roslyn.Microsoft.Build, Roslyn.Microsoft.VisualStudio.ExtensionManager, RoslynDependencies.Microsoft.VisualStudio.Diagnostics.PerformanceProvider and RoslynDependencies.Microsoft.VisualStudio.CodeAnalysis.Sdk.UI.

* Remove workaround now that new NuGet packages have been uploaded to nuget.org

* Remove preprocessor directive MSBUILD12 and its references