Rename UserAction to Signature

We had a big debate about this [here](https://github.com/octokit/octokit.net/pull/167/files#r7484585), but I realised this is what LibGit2Sharp
calls it. https://github.com/libgit2/libgit2sharp/blob/vNext/LibGit2Sharp/Signature.cs

/cc @shiftkey @ninjanye