Create the project structure

Some important bits about this:

• Cross platform code always goes in the same directory as csproj files
• Platform specific code always goes in a folder (but don't use that for the namespace name)
• Platform names are whatever NuGet calls the platform, this will come in handy later

TODO

• Create VS Solution
• Create Windows-specific csprojs
• Figure out where "Xamarin.Mac Library" went to