Added project rubberband-dll to create Windows DLL files (32-bit and 64-bit)

that expose the Rubber Band API as a flat API that can be imported through
runtime linking.

Added project rubberband-sharp to provide a .NET wrapper of the rubberband-dll
files, dynamically dispatching to the 32- or 64-bit DLL based on the process
platform. Configured NuGet package creation, with compatibility for both
packages.config and PackageReference style consumers.

rubberband-dll/dllmain.cpp

@@ -0,0 +1,19 @@
+// dllmain.cpp : Defines the entry point for the DLL application.
+#include "stdafx.h"
+
+BOOL APIENTRY DllMain( HMODULE hModule,
+                       DWORD  ul_reason_for_call,
+                       LPVOID lpReserved
+                     )
+{
+    switch (ul_reason_for_call)
+    {
+    case DLL_PROCESS_ATTACH:
+    case DLL_THREAD_ATTACH:
+    case DLL_THREAD_DETACH:
+    case DLL_PROCESS_DETACH:
+        break;
+    }
+    return TRUE;
+}
+