Sign assemblies

I am using octokit.net in a Visual Studio Extension, but extensions must be strongly named. Therefore, all assemblies that my extension references must also be strongly named.

I added a strong name key file and used it to sign all of the assemblies. I modified the InternalsVisibleTo accordingly.

I also added two rules to Octokit.ruleset that were causing the build to fail with dozens of errors in Visual Studio 2013. New rules or better detection? Either way, they should be easy to fix and re-add.

- CA1811: Avoid uncalled private code
- CA1812: Avoid uninstantiated internal classes

Everything builds and all unit tests pass.