API interface refactor, use new v3 log-sessions API, rename tail -p to -d

I refactored `func APIReq()` a bit to make it easier to for the caller to customize the HTTP request if they need to. I intend to use this from the the releases command with the [API's Range headers](https://devcenter.heroku.com/articles/platform-api-reference#ranges) in order to get releases in descending order, and to limit the number of releases returned.

I didn't yet make that change because I'm waiting for the outcome of #72 as there will be a conflict.

I also added the new v3 log-sessions API to fix #73, and changed the tail `-p` argument to `-d` to better reflect the dyno noun. That is consistent with the `dynos` command in #72 imo, but both are open for discussion.