Misc improvements for Test-Package with Windows Sandbox

[Thilas]

On top of chocolatey-community#26, here a some changes I'd like to suggest:

• change with parameter sets and parameters to make is clearer what the options are
• change the way Test-Package determines the package name and version from a nupkg file (this could fail if the file is not named properly or if the package name ends with numbers)
• use throw to ensure errors are properly raised & better error management overall
• replace Test-Package.cmd by a Test-Package.ps1 in Sandbox to simplify how the test is setup (closer to Vagrant)
• minimize Windows Sandbox windows as soon as possible to keep the focus on the current terminal

What do you think?