Support for ReSharper 8.1

Supports both 8.0 and 8.1 for each package. Means building 8.0 and 8.1 at the same time. Since the 8.1 SDK is now distributed by nuget, that means it alters the packages.config file, but that's already in use for the 8.0 project. So, the 8.1 projects are in a ReSharper81 folder. Not ideal, but will be fixed in nuget 2.8, which allows for packages.projectname.config.

Alternatively, could commit the 8.0 binaries, build 8.1 and bundle both into the package. Let me know if you'd prefer this, and I can update the PR.