Adds UwpTool.install and UwpTool.uninstall methods. Refactors the
PowerShell-based install code to move the powershell-related bits out of
the Device class and into UwpTool so that when we swap out the
PowerShell-based install for the uwptool-based install, it's transparent
to the WindowsUWPDevice class.

Adds implementations for:
* WindowsUWPDevice.isAppInstalled
* WindowsUWPDevice.uninstallApp

Refactors:
* WindowsUWPDevice.installApp