Turned on NuGet Package Restore

Added the necessary .gitignore entries to exclude the nuget
packages themselves from the repository but still allow
Visual Studio/msbuild to download and install the required
packages at build time.

modified:   .gitignore
new file:   source/.nuget/NuGet.Config
new file:   source/.nuget/NuGet.targets
modified:   source/AspNetIdentity/AspNetIdentity.csproj
modified:   source/Core/Assets/Scripts/Bundle.js
modified:   source/Core/Core.csproj
modified:   source/Host/Host.csproj
modified:   source/MembershipReboot/MembershipReboot.csproj
modified:   source/Thinktecture.IdentityManager.sln
new file:   source/packages/repositories.config