addded fxn for hitting the contributor stats endpoint

Just a small addition to be able to hit the contributor stats endpoint at https://developer.github.com/v3/repos/statistics/#contributors For example:

```coffee
library("github")
clientid <- getOption("github_client_id")
clientsecret <- getOption("github_client_secret")
ctx <- interactive.login(client_id = clientid, client_secret = clientsecret)
get.repository.contributors.stats("rOpenSci", "taxize", ctx)
```