File structure changes for TeamCity build server setup

- Build server will build NuGet packages from now on, so removing the
nuget directory which had all the old packages in it
- Moving the nuspec file to the project directory
- Moving the content and tools nuget pacakge directories into the
project directory within a _nuget directory
- Created new MongoDb, RavenDb, CouchDb and Xml nuspec files so we can
publish unstable versions to NuGet if we choose