Improve release script

#### Auto-detect upstream remote

If a remote named `upstream` exists, use it, otherwise use `origin`.

#### Update changelog

As discussed at #1205, update the changelog using [`github-changes`](/lalitkapoor/github-changes).  The changelog output will not be exactly the same until lalitkapoor/github-changes#40 and lalitkapoor/github-changes#41 are merged.

*[ Edit: Fixes #1206 ]*