This change updates the StartVS script in the following ways:
* Ported batch file to a PowerShell script. The batch file is still
there. It just runs the PowerShell script.
* Added a `-chooseVS` switch that displays a menu of Visual Studio
instances that are installed and allows the user to pick one to
launch.
* Added an `-includeRoslynDeps` switch that sets an environment variable
that controls whether Roslyn dependencies are included with the
Razor extension or not.
Dustin Campbell