Made sure AutofacMiddleware can only be registered once.
Merge branch 'master' of https://github.com/autofac/Autofac

Conflicts:
	Core/Source/Autofac.Integration.Owin/AutofacAppBuilderExtensions.cs
Fixed merge conflict.
Added AppSettingsModule to Autofac.Configuration based on Paul Stovell's "Convention-based Configuration for Autofac" blog post (http://paulstovell.com/blog/convention-configuration).
Minor cleanup and comment fixes.