Allow NuGet package to be build with module in root

[gpduck]

PowerShellGet packages are currently built expecting the module to be in the root of the NuGet package, so this change allows the build script to specify a custom base for the target in the NuSpec so it can build either Chocolatey or PowerShellGet style packages.

I've left the default behavior to match the current folder structure, but a PowerShellGet module can be built using:

Invoke-PSake .\build.psake.ps1 -properties @{targetBase = "" }

[dlwyatt]

What does this gain us? PowerShellGet takes care of its own NuGet packaging behind the scenes when you run Publish-Module; I'm not aware of a scenario where we have to do that ourselves.

[gpduck]

Flexibility? I'm actually trying to setup my own builds internally at work
but I can't use the project's psake script because I need my output in the
other format. I was trying to use PowerShellGet as a reason that it would
apply to people outside my company.

[nohwnd]

@gpduck @dlwyatt currently we push to NuGet, Chocolatey and PowerShellGallery and as far as I know all the packages work just fine. But if this works without changing the configuration of the build server I see no problem merging it.