Add credential provider for NuGet feeds.

Only supports credentials stored in NuGet.config under
the user's profile:

nuget sources update -name MyFeed -username user -password pass

src/AddIns/Misc/PackageManagement/Project/PackageManagement.csproj

@@ -245,6 +245,7 @@
     <Compile Include="Src\Scripting\NullGlobalMSBuildProjectCollection.cs" />
     <Compile Include="Src\Scripting\RunAllProjectPackageScriptsAction.cs" />
     <Compile Include="Src\ServiceWithWorkbenchOwner.cs" />
+    <Compile Include="Src\SharpDevelopCredentialProvider.cs" />
     <Compile Include="Src\UpdatePackagesAction.cs" />
     <Compile Include="Src\UpdatePackagesActionFactory.cs" />
     <Compile Include="Src\UpdateSolutionPackagesAction.cs" />

src/AddIns/Misc/PackageManagement/Project/Src/PackageManagementServices.cs

@@ -3,6 +3,7 @@
 
 using System;
 using ICSharpCode.PackageManagement.Scripting;
+using NuGet;
 
 namespace ICSharpCode.PackageManagement
 {
@@ -41,6 +42,17 @@ namespace ICSharpCode.PackageManagement
 			resetPowerShellWorkingDirectory = new ResetPowerShellWorkingDirectoryOnSolutionClosed(projectService, ConsoleHost);
 			var consolePackageActionRunner = new ConsolePackageActionRunner(ConsoleHost, packageActionsToRun);
 			packageActionRunner = new PackageActionRunner(consolePackageActionRunner, packageManagementEvents);
+			
+			InitializeCredentialProvider();
+		}
+		
+		static void InitializeCredentialProvider()
+		{
+			ISettings settings = Settings.LoadDefaultSettings(null);
+			var packageSourceProvider = new PackageSourceProvider(settings);
+			var credentialProvider = new SettingsCredentialProvider(new SharpDevelopCredentialProvider(), packageSourceProvider);
+			
+			HttpClient.DefaultCredentialProvider = credentialProvider;
 		}
 		
 		public static PackageManagementOptions Options {

src/AddIns/Misc/PackageManagement/Project/Src/SharpDevelopCredentialProvider.cs

@@ -0,0 +1,18 @@
+// Copyright (c) AlphaSierraPapa for the SharpDevelop Team (for details please see \doc\copyright.txt)
+// This code is distributed under the GNU LGPL (for details please see \doc\license.txt)
+
+using System;
+using System.Net;
+using NuGet;
+
+namespace ICSharpCode.PackageManagement
+{
+	public class SharpDevelopCredentialProvider : ICredentialProvider
+	{
+		public ICredentials GetCredentials(Uri uri, IWebProxy proxy, CredentialType credentialType, bool retrying)
+		{
+			return null;
+		}
+	}
+}
+