Git-when-merged

This release incorporates an old error-reporting fix plus the boilerplate needed to release the project to PyPI (from which the project can now be installed; see the README for instructions).

This version adds some new options:
- `--show-commit`: Display only the SHA-1 of the merge commit. Exit with a nonzero exit code if the commit was not merged via a merge commit. This option is meant to make it convenient to call `git when-merged` from scripts.
- `--show-branch`: Display all of the commits that were merged at the same time as the specified commit (e.g., the whole pull request branch). This option also interacts nicely with `--log` and `--visualize`.
- `--describe` and `--describe-contains`: Describe the merge commit in terms of nearby tags (i.e., pass it through `git describe` or `git describe --contains`).

This is a bugfix release for version 1.1.0.

Add two new options:
- `--recursive`/`-r`: If the commit was merged via multiple merges, find all of them.
- `--log`/`-l` : Show the log message for the merge commit (i.e., without the diff). This seems more useful than `--diff`.