Moved MVC dependency to its own assembly.
Updates build and nuspec files.
Fixes build break in documentation build.
Merge branch 'v4.3' into MVCDependency
Fixes a bunch of build warnings and an error.
Fixes NuGet build breaks.
Refactored to reverse the breaking change, and instead add AsActionResultMvc5
Adds optional NuGet package DotNetOpenAuth.Mvc5.
By installing this optional package, MVC5 customers can change their
AsActionResult() extension methods to instead use:
AsActionResultMvc5().
All other code remains the same and it fixes the MVC 5 break for DNOA.

Fixes #307
Merge branch 'MVCDependency' into v4.3
Fixes NuGet package build break.