Add support for Wix v4 codebase

[Wautvda]

Currently Wix Toolset v4 isn't supported with this release. When building you get the following exception
C:\Program Files (x86)\WiX Toolset v4.0\bin\candle.exe -out obj\Debug\ -dDebug -arch x86 -ext ..\packages\PowerShellWixExtension.2.0.1\tools\lib\PowerShellWixExtension.dll Product.wxs candle.exe(0,0): error CNDL0001: Unable to load one or more of the requested types. Retrieve the LoaderExceptions property for more information.

Using the v3 codebase this isn't a problem.

[flcdrg]

Sounds like a major release is due that includes this