NuGet Symbol Packages

This PR addresses issue #224.

The *MakeRelease.ps1* script has been altered so that it generates NuGet symbol packages in addition to library packages. Only relevant *.cs* files are included in the symbol package *src* directories.

Note that symbol packages are generated even for those packages that don't have source (such as reactiveui). In such cases, the symbol package is empty just like the library package.

I normally use MSBuild to do this kind of thing, so am unaccustomed to some aspects of Powershell. In particular, I was surprised by how limited `Copy-Item` is with respect to recursion and inclusions/exclusions. Hence, it was easier to just use robocopy as Paul suggested.