Add a nuspec for creating the NuGet package.
Update path to find built DLL.

TeamCity builds and tests the DLL, then dumps the DLL to the
configuration's artifacts. The NuGet configuration picks up the artifact
and copies it to its workspace at the root directory, so look for the file
here instead.
Place the DLL inside a framework folder.

From the build:

> Issue: Assembly not inside a framework folder.
Description: The assembly 'lib\BobTheBuilder.dll' is placed directly under
'lib' folder. It is recommended that assemblies be placed inside a
framework-specific folder.
Solution: Move it into a framework-specific folder. If
this assembly is targeted for multiple frameworks, ignore this warning.
Fix package ID.