Specify the github name when using an OAuth token

When using an OAuth token, the current API specifies that the [correct usage](https://developer.github.com/v3/auth/#working-with-two-factor-authentication) is to provide the token as your username, and either a blank password, or a password of `x-oauth-basic`.  If I set up my `~/.gitconfig` to reflect this, [line 60 of github.rb](https://github.com/seattlerb/omnifocus-github/blob/18bea00e00f41e58fafd1eaa9beb29b73f2245cd/lib/omnifocus/github.rb#L60) fails, as it tries to retrieve `https://api.github.com/users/#{auth[:user]}`, but `auth[:user]` is my OAuth token.

This PR introduces another configuration option called `name`, to specify the github account name.  Thus, the correct configuration in my case is now:

```
git config --global github.name mutemule
git config --global github.user <oauth token>
git config --global github.password x-oauth-basic
```