Support target framework specific NuGet PowerShell scripts.

Fix AutoMapper NuGet PowerShell scripts not being run.

A NuGet package can have install.ps1 and uninstall.ps1 PowerShell
scripts that target a particular framework based on the tools
subdirectory where they can be placed:

    tools\net40\install.ps1
    tools\net40\uninstall.ps1

Previously the addin would only look in the tools directory for
these scripts. Now it will use the project's target framework to find
the most compatible PowerShell install or uninstall script.

Note that NuGet does not support init.ps1 being target framework
specific and it will only be run if it exists in the tools directory
inside the NuGet package.
Fix compiler warnings.